[NET]: Add missing struct net_device forward decl to skbuff.h

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@redhat.com>

include/linux/skbuff.h

@@ -89,6 +89,8 @@
 #define NET_CALLER(arg) __builtin_return_address(0)
 #endif
 
+struct net_device;
+
 #ifdef CONFIG_NETFILTER
 struct nf_conntrack {
 	atomic_t use;