Avoid adding stdlib (facebook#48468)

Summary:
Pull Request resolved: facebook#48468

Clang will add this code by default at the preprocessor phase. I'd observed a difference in output on sandcastle where it didn't include `stdlib` by default.

This stops `stdlib` being included locally.  **It isn't important wrt tracking user API changes.**

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D67793848

fbshipit-source-id: 0c88aee05a78e2410b308fe10c48db2552b8a148

Author: blakef