Fix codegen for CXX build
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

javache/react-native,facebook/react-native,arthuralee/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,hoangpham95/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,hoangpham95/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,hoangpham95/react-native,pandiaraj44/react-native,pandiaraj44/react-native,facebook/react-native,hoangpham95/react-native,pandiaraj44/react-native,facebook/react-native,myntra/react-native,pandiaraj44/react-native,arthuralee/react-native,myntra/react-native,myntra/react-native,janicduplessis/react-native,janicduplessis/react-native,myntra/react-native,janicduplessis/react-native,javache/react-native,arthuralee/react-native,pandiaraj44/react-native,facebook/react-native,pandiaraj44/react-native,hoangpham95/react-native,pandiaraj44/react-native,facebook/react-native,arthuralee/react-native,janicduplessis/react-native,pandiaraj44/react-native,arthuralee/react-native,javache/react-native,javache/react-native,hoangpham95/react-native,facebook/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,facebook/react-native