Add mg_wakeup()

src/event.h

@@ -25,5 +25,6 @@ enum {
   MG_EV_MQTT_MSG,   // MQTT PUBLISH received        struct mg_mqtt_message *
   MG_EV_MQTT_OPEN,  // MQTT CONNACK received        int *connack_status_code
   MG_EV_SNTP_TIME,  // SNTP time received           uint64_t *epoch_millis
+  MG_EV_WAKEUP,     // mg_wakeup() data received    struct mg_str *data
   MG_EV_USER        // Starting ID for user events
 };