[FR] ability to specify additive platform-specific headers rather than replacements

Platform-specific headers currently are used as a complete replacement for the module headers. Some libraries have a set of common headers and a set of platform-specific headers (ffmpeg, as an example: https://github.com/google/prefab/issues/71).

Adding an `additive_platform_headers: true` option to module.json or possibly an alternative directory name to the library directory would solve this.