aboutdialog: Disable text undo

The text buffers here are not editable, so it
does not make sense to have text undo enabled
for them.

Fixes: #3123

gtk/gtkaboutdialog.c

@@ -2042,6 +2042,8 @@ text_buffer_new (GtkAboutDialog  *about,
   gtk_text_buffer_get_end_iter (buffer, &end_iter);
   gtk_text_buffer_apply_tag (buffer, tag, &start_iter, &end_iter);
 
+  gtk_text_buffer_set_enable_undo (buffer, FALSE);
+
   return buffer;
 }