message: core: add HIL flag to SET_MODE
  This was missed in an earlier commit.
  We need to send this because if the bit is set wrongly, commander will
  refuse to switch modes.
diff: @@ -507,9 +507,12 @@ MavlinkCommands::Result Device::set_flight_mode(FlightMode device_mode)
  void Device::set_flight_mode_async(FlightMode device_mode, command_result_callback_t callback)
  {
  - uint8_t flag_safety_armed = is_armed() ? MAV_MODE_FLAG_SAFETY_ARMED : 0;
  + const uint8_t flag_safety_armed = is_armed() ? MAV_MODE_FLAG_SAFETY_ARMED : 0;
  + const uint8_t flag_hitl_enabled = _hitl_enabled ? MAV_MODE_FLAG_HIL_ENABLED : 0;
  - uint8_t mode = MAV_MODE_FLAG_CUSTOM_MODE_ENABLED | flag_safety_armed;
  + const uint8_t mode = MAV_MODE_FLAG_CUSTOM_MODE_ENABLED
  + | flag_safety_armed
  + | flag_hitl_enabled;
  // Note: the safety flag is not needed in future versions of the PX4 Firmware
  // but want to be rather safe than sorry.
  