[libc] [llvm] [libc] stop including unistd.h in src/ (PR #85971)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

hmm...[a previous run](https://buildkite.com/llvm-project/github-pull-requests/builds/49030#018e5ce9-23c6-46aa-8713-8273405897c2) showed a failure running `ninja libc_unistd_unittests` for libc/test/src/unistd/sysconf_test.cpp. I wonder what could be going wrong there. I can't seem to reproduce locally.

https://github.com/llvm/llvm-project/pull/85971