iptables: Fix connlabel.conf install location

As reported by Danny Rawlins in bug #828, connlabel.conf is
unconditionally installed in /etc/xtables instead of using
prefix set at configure time. Fix to use sysconfdir variable.

This closes bugzilla #828.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>