numix-gtk-theme: add update script

pkgs/data/themes/numix/default.nix

···
-{ lib, stdenv, fetchFromGitHub, sass, glib, libxml2, gdk-pixbuf
+{ lib
+, stdenv
+, fetchFromGitHub
+, sass
+, glib
+, libxml2
+, gdk-pixbuf
 , gtk-engine-murrine
+, gitUpdater
 }:
 
 stdenv.mkDerivation rec {
···
     substituteInPlace Makefile --replace '$(DESTDIR)'/usr $out
     patchShebangs .
   '';
+
+  passthru.updateScript = gitUpdater { };
 
   meta = {
     description = "Modern flat theme with a combination of light and dark elements (GNOME, Unity, Xfce and Openbox)";