Add check for CONFIG_IP_NF_NAT to kernel check

This appears to be missing. NAT (via iptables) is required in order to
setup Docker networking.

Signed-off-by: Charlie Drage <charlie@charliedrage.com>

contrib/check-config.sh

@@ -186,7 +186,7 @@ flags=(
 	VETH BRIDGE BRIDGE_NETFILTER
 	NF_NAT_IPV4 IP_NF_FILTER IP_NF_TARGET_MASQUERADE
 	NETFILTER_XT_MATCH_{ADDRTYPE,CONNTRACK,IPVS}
-	NF_NAT NF_NAT_NEEDED
+	IP_NF_NAT NF_NAT NF_NAT_NEEDED
 
 	# required for bind-mounting /dev/mqueue into containers
 	POSIX_MQUEUE