Fix missing header search path for React-featureflags in React-CoreModules (#56592)

RCTRedBox.mm in React-CoreModules imports <react/featureflags/ReactNativeFeatureFlags.h>
(added in #56574, RedBox 2.0 iOS backport). With use_frameworks!, this file fails
to build with:

  'react/featureflags/ReactNativeFeatureFlags.h' file not found
  (Did not find header 'featureflags/ReactNativeFeatureFlags.h' in framework 'react')

React-CoreModules.podspec declared the React-featureflags dependency using plain
s.dependency, which only registers the pod. The React-Native-specific
add_dependency helper additionally appends the framework's Headers directory
(React_featureflags.framework/Headers) to HEADER_SEARCH_PATHS, which is required
for Clang to resolve <react/featureflags/...> under use_frameworks!.

This aligns React-featureflags with how React-debug, React-runtimeexecutor,
React-jsinspector*, React-utils, React-NativeModulesApple, ReactCommon and
React-RCTFBReactNativeSpec are already declared in the same podspec.

Verified: regenerated React-CoreModules.debug.xcconfig now contains
${PODS_CONFIGURATION_BUILD_DIR}/React-featureflags/React_featureflags.framework/Headers
and the build succeeds.

Author: radko93

packages/react-native/React/CoreModules/React-CoreModules.podspec

@@ -51,7 +51,7 @@ Pod::Spec.new do |s|
   s.dependency "React-Core/CoreModulesHeaders", version
   s.dependency "React-RCTImage", version
   s.dependency "React-jsi", version
-  s.dependency "React-featureflags"
+  add_dependency(s, "React-featureflags")
   s.dependency 'React-RCTBlob'
   add_dependency(s, "React-debug")
   add_dependency(s, "React-runtimeexecutor", :additional_framework_paths => ["platform/ios"])