x11: Add the same message as the wayland backend has

This seems prudent, if we make noise about missing cursors
on Wayland, we should do the same on X11.

gdk/x11/gdkcursor-x11.c

@@ -704,7 +704,10 @@ _gdk_x11_display_get_cursor_for_name (GdkDisplay  *display,
             }
         }
       if (xcursor == None)
-        return NULL;
+        {
+          g_message ("Unable to load %s from the cursor theme", name);
+          return NULL;
+        }
     }
 
   private = g_object_new (GDK_TYPE_X11_CURSOR,