Fixed compilation for LWIP_DNS==1 after changing sys layer

src/include/lwip/api_msg.h

@@ -127,7 +127,7 @@ struct dns_api_msg {
   ip_addr_t *addr;
   /** This semaphore is posted when the name is resolved, the application thread
       should wait on it. */
-  sys_sem_t sem;
+  sys_sem_t *sem;
   /** Errors are given back here */
   err_t *err;
 };