Fix codegen for CXX build

Summary:
Fabric codegen is failing with running without specifying a specific platform. This diff fixes that.

Steps to reproduce:
```
buck build //xplat/js/react-native-github:generated_components-rncore --show-output
```

Error:
```
#include <react/renderer/imagemanager/primitives.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

changelog: [internal] internal

Reviewed By: fkgozali

Differential Revision: D23114403

fbshipit-source-id: 50210e74f2a99ae5a77087988c3323bdacf04128

ReactCommon/react/renderer/imagemanager/BUCK

@@ -24,6 +24,13 @@ rn_xplat_cxx_library(
  exclude = glob(["tests/**/*.cpp"]),
  ),
  header_namespace = "",
+ exported_headers = subdir_glob(
+ [
+ ("", "*.h"),
+ ("platform/cxx", "**/*.h"),
+ ],
+ prefix = "react/renderer/imagemanager",
+ ),
  compiler_flags = [
  "-fexceptions",
  "-frtti",