GCS_MAVLink: add CAMERA_FEEDBACK to enum

libraries/GCS_MAVLink/GCS.h

@@ -49,6 +49,7 @@ enum ap_message {
     MSG_RANGEFINDER,
     MSG_TERRAIN,
     MSG_BATTERY2,
+    MSG_CAMERA_FEEDBACK,
     MSG_RETRY_DEFERRED // this must be last
 };