message: device_impl: make enum explicit
diff: @@ -222,7 +222,7 @@ void DeviceImpl::send_heartbeat(DeviceImpl *self)
  {
  mavlink_message_t message;
  mavlink_msg_heartbeat_pack(_own_system_id, _own_component_id, &message,
  - MAV_TYPE_GCS, 0, 0, 0, 0);
  + MAV_TYPE_GCS, MAV_AUTOPILOT_GENERIC, 0, 0, 0);
  self->send_message(message);
  }
  