GCS_MAVLink: add EKF_STATUS_REPORT to enum

libraries/GCS_MAVLink/GCS.h

@@ -56,6 +56,7 @@ enum ap_message {
     MSG_MOUNT_STATUS,
     MSG_OPTICAL_FLOW,
     MSG_GIMBAL_REPORT,
+    MSG_EKF_STATUS_REPORT,
     MSG_RETRY_DEFERRED // this must be last
 };