hello-wayland: add update script

pkgs/applications/graphics/hello-wayland/default.nix

@@ -1,5 +1,6 @@
 { stdenv, lib, fetchFromGitHub
 , imagemagick, pkg-config, wayland, wayland-protocols
+, unstableGitUpdater
 }:
 
 stdenv.mkDerivation {
@@ -23,6 +24,8 @@ stdenv.mkDerivation {
     runHook postBuild
   '';
 
+  passthru.updateScript = unstableGitUpdater { };
+
   meta = with lib; {
     description = "Hello world Wayland client";
     homepage = "https://github.com/emersion/hello-wayland";