theme-obsidian2: add update script

pkgs/data/themes/obsidian2/default.nix

@@ -2,6 +2,7 @@
 , stdenv
 , fetchurl
 , gtk-engine-murrine
+, gitUpdater
 }:
 
 stdenv.mkDerivation rec {
@@ -26,6 +27,11 @@ stdenv.mkDerivation rec {
     runHook postInstall
   '';
 
+  passthru.updateScript = gitUpdater {
+    url = "https://github.com/madmaxms/theme-obsidian-2";
+    rev-prefix = "v";
+  };
+
   meta = with lib; {
     description = "Gnome theme based upon Adwaita-Maia dark skin";
     homepage = "https://github.com/madmaxms/theme-obsidian-2";