Plane: support do-mount-control via command-long

ArduPlane/GCS_Mavlink.cpp

@@ -1161,6 +1161,12 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
             break;
 #endif
 
+        case MAV_CMD_DO_MOUNT_CONTROL:
+#if MOUNT == ENABLED
+            plane.camera_mount.control(packet.param1, packet.param2, packet.param3, (MAV_MOUNT_MODE) packet.param7);
+#endif
+            break;
+
         case MAV_CMD_MISSION_START:
             plane.set_mode(AUTO);
             result = MAV_RESULT_ACCEPTED;