Mark FantomForcedCloneCommitHook.h as private C++ API

Summary:
Include `react/cxxstableapi/PrivateGuard.h` in `FantomForcedCloneCommitHook.h` to mark it as a private header, matching the sibling `NativeFantomTestSpecificMethods.h`.

Changelog: [Internal]

Differential Revision: D125414094

Author: j-piasecki

packages/react-native/ReactCommon/react/nativemodule/fantomtestspecificmethods/internal/FantomForcedCloneCommitHook.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <react/cxxstableapi/PrivateGuard.h>
+
 #include <react/renderer/core/ShadowNode.h>
 #include <react/renderer/uimanager/UIManager.h>
 #include <react/renderer/uimanager/UIManagerCommitHook.h>