netfilter: nf_tables: nat expression must select CONFIG_NF_NAT

This enables the netfilter NAT engine in first place, otherwise
you cannot ever select the nf_tables nat expression if iptables
is not selected.
Reported-by: Matteo Croce <technoboy85@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/Kconfig

@@ -499,7 +499,7 @@ config NFT_LIMIT
 config NFT_NAT
 	depends on NF_TABLES
 	depends on NF_CONNTRACK
-	depends on NF_NAT
+	select NF_NAT
 	tristate "Netfilter nf_tables nat module"
 	help
 	  This option adds the "nat" expression that you can use to perform