utils: nfnl_osf: use the right nfnetlink lib

If the user specify libnfnetlink_LIBS during the configure, we must use it.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Merge branch 'stable'

Resolve conflict with Nicolas' Dichtel update on utils/Makefile.am
for nfnl_osf.