message: backend: fix warning coming with new AppleClang
diff: @@ -149,7 +149,7 @@ public:
  const dronecore::rpc::telemetry::SubscribeBatteryRequest * /* request */,
  grpc::ServerWriter<rpc::telemetry::BatteryResponse> *writer) override
  {
  - _telemetry.battery_async([this, &writer](dronecore::Telemetry::Battery battery) {
  + _telemetry.battery_async([&writer](dronecore::Telemetry::Battery battery) {
  auto rpc_battery = new dronecore::rpc::telemetry::Battery();
  rpc_battery->set_voltage_v(battery.voltage_v);
  rpc_battery->set_remaining_percent(battery.remaining_percent);
  