gamescope-wsi: depend on libxcb independently of libx11 propagating it

Maybe libxcb should be `optionals enableWsi` since the change didn't
break the gamescope package without Wsi, but i found out it also builds
fine if i move libx11 to `optionals enableWsi` so this is something
the gamescope maintainers have to figure out.

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

···
     pipewire
     hwdata
     xorg.libX11
+    xorg.libxcb
     wayland
     wayland-protocols
     vulkan-loader