netfilter: add explicit Kconfig for NETFILTER_XT_NAT

Paul Bolle reports that 'select NETFILTER_XT_NAT' from the IPV4 and IPV6
NAT tables becomes noop since there is no Kconfig switch for it. Add the
Kconfig switch to resolve this problem.

Fixes: 8993cf8e netfilter: move NAT Kconfig switches out of the iptables scope
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>