Support for mqttv3.1 subprotocol string.

src/websockets.c

@@ -87,6 +87,13 @@ static struct libwebsocket_protocols protocols[] = {
 		0,
 		1,
 	},
+	{
+		"mqttv3.1",
+		callback_mqtt,
+		sizeof(struct libws_mqtt_data),
+		0,
+		1,
+	},
 	{ NULL, NULL, 0, 0 }
 };