flatpak: Pin down the Desmume and Beetle NGP commits

This will help Games to build until the upstream build bugs are solved.

flatpak/libretro-cores/libretro-desmume.json

@@ -18,7 +18,8 @@
             "sources": [
                 {
                     "type": "git",
-                    "url": "https://github.com/libretro/desmume"
+                    "url": "https://github.com/libretro/desmume",
+                    "branch": "09e0f16bf2ab6ffb3267ab27a9135867cf6320f3"
                 },
                 {
                     "type": "file",

flatpak/libretro-cores/libretro-mednafen_ngp.json

@@ -17,7 +17,8 @@
             "sources": [
                 {
                     "type": "git",
-                    "url": "https://github.com/libretro/beetle-ngp-libretro"
+                    "url": "https://github.com/libretro/beetle-ngp-libretro",
+                    "branch": "bc52b15e7e0cba152dad1e8e24f06f3fff0c36a6"
                 },
                 {
                     "type": "file",