glx: Use _Xglobal_lock for protecting extension display list

Avoids double locking glXLock in the X wire to event handlers.
diff --git a/src/glx/glxclient.h b/src/glx/glxclient.h
index 26f2885..4f83305 100644
--- a/src/glx/glxclient.h
+++ b/src/glx/glxclient.h
@@ -557,6 +557,10 @@
  */
 struct __GLXdisplayPrivateRec
 {
+   /* The extension protocol codes */
+   XExtCodes *codes;
+   struct __GLXdisplayPrivateRec *next;
+
     /**
      * Back pointer to the display
      */