Flatpak: Grant access to Gamescope socket for Steam Deck

* See flathub/org.DolphinEmu.dolphin-emu#178
* Bug only occurs on the Steam Deck OLED when using the Vulkan renderer.

scripts/packaging/flatpak/org.duckstation.DuckStation.yaml

@@ -19,6 +19,8 @@ finish-args:
   - "--talk-name=org.freedesktop.ScreenSaver"
   - "--socket=wayland"
   - "--socket=fallback-x11"
+  # Required for Gamescope on Steam Deck OLED when using the Vulkan renderer
+  - --filesystem=xdg-run/gamescope-0:ro
 
 modules:
   # Dependencies.