[libcxx-commits] [libcxx] [libc++] Add missing include to thread.h (PR #208740)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 requested changes to this pull request.

`is_integral` isn't used inside `thread.h` AFAICT.

https://github.com/llvm/llvm-project/pull/208740