Summary:
`JSBigString` is using functions from `unistd.h`, like `getpagesize`, `dup`, `open`, etc. but was not directly including it.
It was being included from inside the glog `logging.h` header, which in turn was getting included by the Folly headers `JSBigString` was using.
This was discovered while building CxxReact with a custom shimmed Glog.
Pull Request resolved: https://github.com/facebook/react-native/pull/22330
Differential Revision: D13115346
Pulled By: shergin
fbshipit-source-id: 9fe4e3b28f74c0ca351ac6308484e375eace4db4
d6d31a4b66