jgmenu: add update script

pkgs/applications/misc/jgmenu/default.nix

···
 , enableXfcePanelApplet ? false
 , xfce
 , gtk3
+, gitUpdater
 }:
 
 stdenv.mkDerivation rec {
···
       wrapProgram $f --prefix PATH : $out/bin
     done
   '';
+
+  passthru.updateScript = gitUpdater { rev-prefix = "v"; };
 
   meta = with lib; {
     homepage = "https://github.com/johanmalm/jgmenu";