Adding EXECUTORCH_CLIENTS visibility to parallel extension (#2768)

Summary:
title.


Reviewed By: mergennachin

Differential Revision: D55505037

Pulled By: ChristianWLang

Author: ChristianWLang

extension/parallel/targets.bzl

@@ -20,6 +20,7 @@ def define_common_targets():
             ],
             visibility = [
                 "//executorch/...",
+                "@EXECUTORCH_CLIENTS",
             ],
             deps = [
                 "//executorch/backends/xnnpack/threadpool:threadpool",