net: Fix warnings in dst_ops.h

include/net/dst_ops.h:28:20: warning: ‘struct sock’ declared inside parameter list
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst_ops.h

@@ -8,6 +8,7 @@ struct dst_entry;
 struct kmem_cachep;
 struct net_device;
 struct sk_buff;
+struct sock;
 
 struct dst_ops {
 	unsigned short		family;