src/utils/Makefile.inc: Add $(libnghttp2_CFLAGS) where needed

The header file src/utils/common/https.h includes the external header
file <nghttp2/nghttp2.h> (when nghttp2 support is enabled by the build
system). This header file is included by src/utils/common/netio.h, which can
in turn be included directly or indirectly by various knot components.

The nghttp2 header files are not guaranteed to be located in the compiler's
default include paths. This dependency is detected by the build system
via PKG_CHECK_MODULES, which populates the automake variable
$(libnghttp2_CFLAGS). These extra CFLAGS must be added to the compiler flags
for any targets that include a header file that results in header file
inclusions of the external dependencies. Otherwise the build will fail when
those external header files cannot be located.

This commit adds $(libnghttp2_CLAGS) to the *_CPPFLAGS for kdig, khost, and
knsupdate.

Without the $(libnghttp2_CFLAGS) fixes in this commit, the build will
fail with an error like:

    Making all in src
    make[1]: Entering directory '/tmp/knot-3.0.0/src'
      GEN      contrib/dnstap/dnstap.pb-c.c
      GEN      contrib/dnstap/dnstap.pb-c.h
    NOTE: Compilation of scanner.c can take several minutes!
    make  all-recursive
    make[2]: Entering directory '/tmp/knot-3.0.0/src'
    make[3]: Entering directory '/tmp/knot-3.0.0/src'
      CC       libdnssec/la-tsig.lo
      CC       utils/kdig/khost-kdig_exec.o
      CC       utils/kdig/khost-kdig_params.o
      CC       utils/kdig/kdig-kdig_exec.o
      CC       utils/kdig/kdig-kdig_main.o
      CC       utils/kdig/kdig-kdig_params.o
      CC       utils/common/libknotus_la-cert.lo
      CC       utils/common/libknotus_la-exec.lo
      CC       utils/common/libknotus_la-hex.lo
      CC       utils/common/libknotus_la-https.lo
      CC       utils/common/libknotus_la-lookup.lo
      CC       utils/common/libknotus_la-msg.lo
    In file included from ./utils/common/netio.h:23,
                     from ./utils/common/exec.h:21,
                     from ./utils/kdig/kdig_params.h:22,
                     from ./utils/kdig/kdig_exec.h:19,
                     from utils/kdig/kdig_exec.c:22:
    ./utils/common/https.h:45:10: fatal error: nghttp2/nghttp2.h: No such file or directory
       45 | #include <nghttp2/nghttp2.h>
          |          ^~~~~~~~~~~~~~~~~~~
    compilation terminated.
    In file included from ./utils/common/netio.h:23,
                     from ./utils/common/exec.h:21,
                     from ./utils/kdig/kdig_params.h:22,
                     from utils/kdig/kdig_params.c:23:
    ./utils/common/https.h:45:10: fatal error: nghttp2/nghttp2.h: No such file or directory
       45 | #include <nghttp2/nghttp2.h>
          |          ^~~~~~~~~~~~~~~~~~~
    compilation terminated.
    make[3]: *** [Makefile:5500: utils/kdig/khost-kdig_exec.o] Error 1
    make[3]: *** Waiting for unfinished jobs....
    make[3]: *** [Makefile:5514: utils/kdig/khost-kdig_params.o] Error 1
    In file included from ./utils/common/netio.h:23,
                     from ./utils/common/exec.h:21,
                     from ./utils/kdig/kdig_params.h:22,
                     from utils/kdig/kdig_main.c:20:
    ./utils/common/https.h:45:10: fatal error: nghttp2/nghttp2.h: No such file or directory
       45 | #include <nghttp2/nghttp2.h>
          |          ^~~~~~~~~~~~~~~~~~~
    compilation terminated.
    make[3]: *** [Makefile:5416: utils/kdig/kdig-kdig_main.o] Error 1
    In file included from ./utils/common/netio.h:23,
                     from ./utils/common/exec.h:21,
                     from ./utils/kdig/kdig_params.h:22,
                     from ./utils/kdig/kdig_exec.h:19,
                     from utils/kdig/kdig_exec.c:22:
    ./utils/common/https.h:45:10: fatal error: nghttp2/nghttp2.h: No such file or directory
       45 | #include <nghttp2/nghttp2.h>
          |          ^~~~~~~~~~~~~~~~~~~
    compilation terminated.
    In file included from ./utils/common/netio.h:23,
                     from ./utils/common/exec.h:21,
                     from ./utils/kdig/kdig_params.h:22,
                     from utils/kdig/kdig_params.c:23:
    ./utils/common/https.h:45:10: fatal error: nghttp2/nghttp2.h: No such file or directory
       45 | #include <nghttp2/nghttp2.h>
          |          ^~~~~~~~~~~~~~~~~~~
    compilation terminated.
    make[3]: *** [Makefile:5402: utils/kdig/kdig-kdig_exec.o] Error 1
    make[3]: *** [Makefile:5430: utils/kdig/kdig-kdig_params.o] Error 1
    make[3]: Leaving directory '/tmp/knot-3.0.0/src'
    make[2]: *** [Makefile:5950: all-recursive] Error 1
    make[2]: Leaving directory '/tmp/knot-3.0.0/src'
    make[1]: *** [Makefile:2046: all] Error 2
    make[1]: Leaving directory '/tmp/knot-3.0.0/src'
    make: *** [Makefile:491: all-recursive] Error 1