selftests: netfilter: fix libmnl pkg-config usage

1. Don't hard-code pkg-config
2. Remove distro-specific default for CFLAGS
3. Use pkg-config for LDLIBS

Fixes: a50a88f0

 ("selftests: netfilter: fix a build error on openSUSE")
Suggested-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>