"xdg-config" permission is ignored by Protontricks

Using https://github.com/flathub/org.gtk.Gtk3theme.Breeze?tab=readme-ov-file#workarounds doesn't work for Protontricks. I use that to fix theming in my GTK3 Flatpak apps.

It gives me this error:
```bash
protontricks (WARNING): Unknown Flatpak file system permission 'xdg-config/gtk-3.0:ro', ignoring.
```