netfilter: nf_nat: support IPv6 in FTP NAT helper

Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
index 91addda..3104494 100644
--- a/net/netfilter/Kconfig
+++ b/net/netfilter/Kconfig
@@ -380,6 +380,11 @@
 	depends on NF_NAT && NF_CT_PROTO_SCTP
 	select LIBCRC32C
 
+config NF_NAT_FTP
+	tristate
+	depends on NF_CONNTRACK && NF_NAT
+	default NF_NAT && NF_CONNTRACK_FTP
+
 endif # NF_CONNTRACK
 
 # transparent proxy support