dnssec: prevent duplicate PKCS11 module loading

src/dnssec/Makefile.am

src/dnssec/lib/crypto.c

src/dnssec/lib/dnssec/error.h

src/dnssec/lib/error.c

src/dnssec/lib/keystore/pkcs11.c

src/dnssec/lib/p11/p11.c

src/dnssec/lib/p11/p11.h