message: telemetry: fix compile warning
  the warning was: control reaches end of non-void function
diff: @@ -254,8 +254,10 @@ const char *Telemetry::flight_mode_str(FlightMode flight_mode)
  case FlightMode::OFFBOARD:
  return "Offboard";
  case FlightMode::UNKNOWN:
  + default:
  return "Unknown";
  }
  +
  }
  void Telemetry::health_async(health_callback_t callback)
  