flatpak: Add sandbox hole to inhibit screensaver

Current Godot releases support using org.freedesktop.ScreenSaver to
inhibit the screensaver, but do not support using the Inhibit portal.

I have implemented Inhibit portal support upstream, but it is awaiting
review; and since it missed the 4.5 feature freeze it will not be part
of a Godot release until 4.6, which is probably close to a year away.

godotengine/godot#108704

Author: wjt

org.endlessaccess.threadbare.yml

@@ -13,6 +13,7 @@ finish-args:
 - --socket=fallback-x11
 - --socket=pulseaudio
 - --device=all
+- --talk-name=org.freedesktop.ScreenSaver
 modules:
 - name: threadbare
   buildsystem: simple