try to avoid conflict with other Display tyepdef's.

git-svn-id: http://root.cern.ch/svn/root/trunk@48832 27541ba8-7e3a-0410-8455-c3a389f83636

graf3d/gl/inc/TX11GL.h

@@ -28,7 +28,7 @@
 #if !defined(__CLING__)
 #include <GL/glx.h>
 #else
-struct Display;
+typedef struct _XDisplay Display;
 struct XVisualInfo;
 #endif