qt6.qtdeclarative: add sigtool on Darwin

Needed since 6.9

pkgs/development/libraries/qt-6/modules/qtdeclarative/default.nix

@@ -5,6 +5,7 @@
   qtshadertools,
   qtsvg,
   openssl,
+  darwin,
   stdenv,
   lib,
   pkgsBuildBuild,
@@ -23,6 +24,10 @@ qtModule {
   ];
   strictDeps = true;
 
+  nativeBuildInputs = lib.optionals stdenv.isDarwin [
+    darwin.sigtool
+  ];
+
   patches = [
     # invalidates qml caches created from nix applications at different
     # store paths and disallows saving caches of bare qml files in the store.