mojave-gtk-theme: add update script

pkgs/data/themes/mojave/default.nix

@@ -16,6 +16,7 @@
 , opacityVariants ? [] # default to all
 , themeVariants ? [] # default to MacOS blue
 , wallpapers ? false
+, gitUpdater
 }:
 
 let
@@ -114,6 +115,8 @@ stdenv.mkDerivation rec {
     runHook postInstall
   '';
 
+  passthru.updateScript = gitUpdater {inherit pname version; };
+
   meta = with lib; {
     description = "Mac OSX Mojave like theme for GTK based desktop environments";
     homepage = "https://github.com/vinceliuice/Mojave-gtk-theme";