ANDROID: base-cfg: enable CONFIG_IP_NF_NAT

IP_NF_TARGET_{MASQUERADE,NETMAP,REDIRECT} configs,
already enabled in android-base.cfg for tethering,
are of no use if CONFIG_IP_NF_NAT is not enabled.

Don't rely on platform config for that and enable
CONFIG_IP_NF_NAT in android-base.cfg as well.

Change-Id: Ic72bcebbd925b142b09539466bf963188c83108a
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>