gamescope: depend on wayland-scanner

Otherwise, wayland-scanner would be picked up from the wayland in
buildInputs, which isn't cross-friendly and will stop working when we
split wayland-scanner into a separate package.

pkgs/by-name/ga/gamescope/package.nix

@@ -12,6 +12,7 @@
 , vulkan-headers
 , wayland
 , wayland-protocols
+, wayland-scanner
 , libxkbcommon
 , glm
 , gbenchmark
@@ -99,6 +100,7 @@ stdenv.mkDerivation (finalAttrs: {
   ] ++ lib.optionals enableExecutable [
     makeBinaryWrapper
     glslang
+    wayland-scanner
   ];
 
   buildInputs = [