alsa-plugins: provide update script

pkgs/by-name/al/alsa-plugins/package.nix

@@ -9,6 +9,7 @@
   libogg,
   libpulseaudio,
   speexdsp,
+  directoryListingUpdater,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -31,6 +32,10 @@ stdenv.mkDerivation (finalAttrs: {
     speexdsp
   ];
 
+  passthru.updateScript = directoryListingUpdater {
+    url = "https://alsa-project.org/files/pub/plugins/";
+  };
+
   meta = {
     description = "Various plugins for ALSA";
     homepage = "http://alsa-project.org/";