Remove forward declaration of struct lwip_sock

The forward declaration was added in commit
e2c2afbbe0 for use in struct
lwip_setgetsockopt_data, but became dead after commit
5d2e93e5f0 where the sock pointer was
removed from lwip_setgetsockopt_data

src/include/lwip/sockets.h

@@ -108,8 +108,6 @@ struct sockaddr_storage {
 typedef u32_t socklen_t;
 #endif
 
-struct lwip_sock;
-
 #if !defined(iovec)
 struct iovec {
   void  *iov_base;