Add back DO_CHANGE_SPEED

src/FirmwarePlugin/PX4/PX4FirmwarePlugin.cc

@@ -220,6 +220,7 @@ QList<MAV_CMD> PX4FirmwarePlugin::supportedMissionCommands(void)
          << MAV_CMD_DO_VTOL_TRANSITION << MAV_CMD_NAV_VTOL_TAKEOFF << MAV_CMD_NAV_VTOL_LAND
          << MAV_CMD_DO_DIGICAM_CONTROL
          << MAV_CMD_DO_SET_SERVO
+         << MAV_CMD_DO_CHANGE_SPEED
          << MAV_CMD_NAV_PATHPLANNING;
     return list;
 }