src: display default directory for file inclusion in -h/--help

src: display default directory for file inclusion in -h/--help

If no explicit relative or absolute path is enforced by the user, nft
relies on either -I/--includepath or the default include directory that
is set at compile time.

Given most of our users will rely on packaged versions of nft, provide a
way to display the location of this default includepath directory.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>