Preserve header subdirectory structure in React-debug pod

Set header_mappings_dir unconditionally in React-debug.podspec so that
headers in subdirectories (like redbox/) retain their directory structure
when CocoaPods copies them, regardless of whether USE_FRAMEWORKS is set.

Without this, static library builds flatten all headers into the
header_dir ("react/debug"), making imports like
`<react/debug/redbox/RedBoxErrorParser.h>` fail because the "redbox/"
subdirectory is lost.

Author: motiz88

packages/react-native/ReactCommon/react/debug/React-debug.podspec

@@ -28,6 +28,7 @@ Pod::Spec.new do |s|
   s.source_files           = podspec_sources("**/*.{cpp,h}", "**/*.h")
   s.exclude_files          = "**/tests/**/*.{cpp,h}"
   s.header_dir             = "react/debug"
+  s.header_mappings_dir    = "../.."
   s.pod_target_xcconfig    = { "CLANG_CXX_LANGUAGE_STANDARD" => rct_cxx_language_standard(),
                                "DEFINES_MODULE" => "YES" }