iptables: link against libnetfilter_conntrack

Linking currently fails in --enable-static case:

../extensions/libext.a(libxt_connlabel.o): In function `connlabel_get_name':
iptables/extensions/libxt_connlabel.c:57: undefined reference to `nfct_labelmap_get_name'
[..]
It's libxtables.la(libxt_connlabel.o) using libnetfilter_conntrack.

If libnetfilter_conntrack is not found, @libnetfilter_conntrack_CFLAGS@
and @libnetfilter_conntrack_LIBS@ (and their ${} ones) should be empty,
therefore producing no harm to include unconditionally.

Reported-and-tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Florian Westphal <fw@strlen.de>