undo mistake

src/xrt/include/xrt/xrt_tracking.h

@@ -57,6 +57,9 @@ enum xrt_tracking_type
 
 	// The device(s) are tracked by external SLAM
 	XRT_TRACKING_TYPE_EXTERNAL_SLAM,
+
+	// Something else
+	XRT_TRACKING_TYPE_EXTERNAL,
 };
 
 /*!