Cover react/nativemodule/fantomtestspecificmethods with Stable API guards

Summary:
Marks `react/nativemodule/fantomtestspecificmethods` as a private target under the C++ Stable API: `NativeFantomTestSpecificMethods.h` includes `<react/cxxstableapi/PrivateGuard.h>`, and BUCK and CMake depend on `cxxstableapi`. The guard is inert unless `RN_STRICT_API` is defined, so there is no behavior change.

Changelog: [Internal]

Differential Revision: D125185297

Author: j-piasecki

packages/react-native/ReactCommon/react/nativemodule/fantomtestspecificmethods/CMakeLists.txt

@@ -17,6 +17,7 @@ target_include_directories(react_nativemodule_fantomspecificmethods PRIVATE ${CM
 target_link_libraries(react_nativemodule_fantomspecificmethods
         react_codegen_rncore
         react_cxxreact
+        react_cxxstableapi
         react_renderer_bridging
         react_renderer_core
         react_renderer_graphics

packages/react-native/ReactCommon/react/nativemodule/fantomtestspecificmethods/NativeFantomTestSpecificMethods.h

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