xournalpp: add update script

pkgs/by-name/xo/xournalpp/package.nix

@@ -26,6 +26,7 @@
   # plugins
   withLua ? true,
   lua5_3,
+  nix-update-script,
 }:
 
 stdenv.mkDerivation rec {
@@ -79,6 +80,8 @@ stdenv.mkDerivation rec {
     )
   '';
 
+  passthru.updateScript = nix-update-script { };
+
   meta = {
     description = "Xournal++ is a handwriting Notetaking software with PDF annotation support";
     homepage = "https://xournalpp.github.io/";