distro: add new libmnl build dep for kxdpgun

merged

mentioned in commit 1e246708

The build still fails on leap15 even with this change:

[   66s] utils/kxdpgun/ip_route.c:25:10: fatal error: libmnl/libmnl.h: No such file or directory
[   66s]  #include <libmnl/libmnl.h>
[   66s]           ^~~~~~~~~~~~~~~~~
[   66s] compilation terminated.

_log

@dsalzman Contents of libmnl-devel on Leap 15.2 - I suspect it's related:

d64921bf2bc:/ # rpm -ql libmnl-devel
/usr/include/libmnl
/usr/include/libmnl/libmnl
/usr/include/libmnl/libmnl/libmnl.h
/usr/lib64/libmnl.so
/usr/lib64/pkgconfig/libmnl.pc
fd64921bf2bc:/ # cat /usr/lib64/pkgconfig/libmnl.pc
# libmnl pkg-config file

prefix=/usr
exec_prefix=/usr
libdir=/usr/lib64
includedir=/usr/include/libmnl

Name: libmnl
Description: Minimalistic Netlink communication library
URL: http://netfilter.org/projects/libmnl/
Version: 1.0.4
Requires:
Conflicts:
Libs: -L${libdir} -lmnl
Cflags: -I${includedir}