Cover cxxreact:messagequeuethread with Stable API guards

Summary:
Classifies `cxxreact:messagequeuethread` as a "for frameworks" target under the three-tier C++ stable API visibility model. Consumers that opt into `RN_STRICT_API` now get a warning if they include its headers directly, which they can acknowledge with `RN_ALLOW_FRAMEWORKS`; without that flag the guards are inert, so no existing build changes behaviour.

Changelog: [Internal]

Differential Revision: D119163883

Author: Jakub Piasecki

packages/react-native/ReactCommon/cxxreact/MessageQueueThread.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <react/cxxstableapi/FrameworksGuard.h>
+
 #include <functional>
 
 namespace facebook::react {