[NETFILTER] x-tables: Missing linux/ipv6.h includes.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/xt_length.c b/net/netfilter/xt_length.c
index ab6c710..39c8fae 100644
--- a/net/netfilter/xt_length.c
+++ b/net/netfilter/xt_length.c
@@ -8,6 +8,7 @@
 
 #include <linux/module.h>
 #include <linux/skbuff.h>
+#include <linux/ipv6.h>
 #include <net/ip.h>
 
 #include <linux/netfilter/xt_length.h>