kernel: 3.18: Fix kmod-ipt-nat

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>

SVN-Revision: 43212