The 3.18 kernel introduced new Kconfig options for the xt_nat and iptable_nat
kernel modules, that both belong to the ipt_nat kernel package.

Enable this new options.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43212 3c298f89-4303-0410-b956-a3cf2f4a3e73