srb2: add startupWMClass to desktop item

Fixes an issue where the srb2 icon wasn't being used after launching
the application.

pkgs/by-name/sr/srb2/package.nix

@@ -96,6 +96,7 @@ stdenv.mkDerivation (finalAttrs: {
       desktopName = name;
       genericName = name;
       categories = [ "Game" ];
+      startupWMClass = ".srb2-wrapped";
     })
   ];