allow to use MQTT for discovery of IPs if mDNS is no option (#3887)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

esphome/components/mqtt/mqtt_client.h

@@ -251,6 +251,8 @@ class MQTTClientComponent : public Component {
   void set_on_disconnect(mqtt_on_disconnect_callback_t &&callback);
 
  protected:
+  void send_device_info_();
+
   /// Reconnect to the MQTT broker if not already connected.
   void start_connect_();
   void start_dnslookup_();