feat: Update GameMode to upstream 1.8.2 (#413)

Updates GameMode to use the original repository instead of my fork since
[my fix](FeralInteractive/gamemode#385) has been
published in GameMode since [release
1.8](https://github.com/FeralInteractive/gamemode/releases/tag/1.8).

snap/snapcraft.yaml

@@ -309,8 +309,8 @@ parts:
 
   gamemode:
     after: [meson-deps]
-    source: https://github.com/ashuntu/gamemode.git
-    source-branch: "add-snap-support"
+    source: https://github.com/FeralInteractive/gamemode.git
+    source-tag: "1.8.2"
     plugin: meson
     organize:
       snap/steam/current/usr: usr
@@ -320,9 +320,12 @@ parts:
       - libsystemd-dev
       - pkg-config
       - libdbus-1-dev
+    stage-packages:
+      - libinih1
     prime:
-      - usr/bin/gamemoderun
+      - usr/bin/gamemode*
       - usr/lib/*/libgamemode*.so.*
+      - usr/lib/*/libinih.so.*
 
   debug-tools:
     plugin: nil