[api] Fix compilation issue with macOS

Fixes the following error caught by the Node.js CI:

../deps/v8/src/api.cc:8943:10:
  error: no viable conversion from returned value of type
  'unique_ptr<v8::internal::MicrotaskQueue, default_delete<v8::internal::MicrotaskQueue>>'
  to function return type
  'unique_ptr<v8::MicrotaskQueue, default_delete<v8::MicrotaskQueue>>'

  return microtask_queue;
         ^~~~~~~~~~~~~~~

Change-Id: Ic09dab46bb8f87a2b3b59f5836e0883bfe0e9681
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627533
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61789}

src/api/api.cc

@@ -8536,7 +8536,8 @@ std::unique_ptr<MicrotaskQueue> MicrotaskQueue::New(Isolate* isolate,
   auto microtask_queue =
       i::MicrotaskQueue::New(reinterpret_cast<i::Isolate*>(isolate));
   microtask_queue->set_microtasks_policy(policy);
-  return microtask_queue;
+  std::unique_ptr<MicrotaskQueue> ret(std::move(microtask_queue));
+  return ret;
 }
 
 MicrotasksScope::MicrotasksScope(Isolate* isolate, MicrotasksScope::Type type)