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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> hmm...a previous run 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.

Perhaps the inclusion of <unistd.h> in libc/test/src/unistd/sysconf_test.cpp is getting a different value for _SC_PAGESIZE than the one we provide.

We define this value to 1, but glibc's it's 30. Bionic defines this to 0x0027 (39).

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