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>
diff --git a/utils/Makefile.am b/utils/Makefile.am
index f1bbfc5..66f5c5c 100644
--- a/utils/Makefile.am
+++ b/utils/Makefile.am
@@ -7,4 +7,4 @@
 sbin_PROGRAMS = nfnl_osf
 pkgdata_DATA = pf.os
 
-nfnl_osf_LDADD = -lnfnetlink
+nfnl_osf_LDADD = ${libnfnetlink_LIBS}