add deprecated warning for loop in async mode

fix sync mode

src/WebSocketsServer.h

@@ -42,7 +42,14 @@ public:
         ~WebSocketsServer(void);
 
         void begin(void);
+#if (WEBSOCKETS_NETWORK_TYPE != NETWORK_ESP8266_ASYNC)
         void loop(void);
+#else
+        // Async interface not need a loop call
+        void loop(void) __attribute__ ((deprecated)) {
+
+        }
+#endif
 
         void onEvent(WebSocketServerEvent cbEvent);