Summary:
unistd.h isn't a header available in the windows SDK, so we can't include it from react-native-windows.
I moved the usage of dup, to JSBigString.cpp in a previous PR, so this header should only be needed in the cpp file, not the header. (And react-native-windows doesn't use the cpp file)
## Changelog
[Internal] [Fixed] - Header cleanup
Pull Request resolved: https://github.com/facebook/react-native/pull/25107
Differential Revision: D15602265
Pulled By: cpojer
fbshipit-source-id: 6a62bf8fe6758e400810f37834e8646485120d71
REDMOND\acoates
7f489b63f2