netfilter: remove NFPROTO_DECNET

Decnet has been removed. so no need to reserve space in arrays for it.
Signed-off-by: Florian Westphal <fw@strlen.de>

include/uapi/linux/netfilter.h

@@ -63,7 +63,9 @@ enum {
 	NFPROTO_NETDEV =  5,
 	NFPROTO_BRIDGE =  7,
 	NFPROTO_IPV6   = 10,
+#ifndef __KERNEL__ /* no longer supported by kernel */
 	NFPROTO_DECNET = 12,
+#endif
 	NFPROTO_NUMPROTO,
 };