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: NPaul Bolle <pebolle@tiscali.nl>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/Kconfig

@@ -766,6 +766,14 @@ config NETFILTER_XT_TARGET_MARK
 	(e.g. when running oldconfig). It selects
 	CONFIG_NETFILTER_XT_MARK (combined mark/MARK module).
 
+config NETFILTER_XT_NAT
+	tristate '"SNAT and DNAT" targets support'
+	depends on NF_NAT
+	---help---
+	This option enables the SNAT and DNAT targets.
+
+	To compile it as a module, choose M here. If unsure, say N.
+
 config NETFILTER_XT_TARGET_NETMAP
 	tristate '"NETMAP" target support'
 	depends on NF_NAT