Fix vfork test strcmp buildbot failure (#84224)

The buildbot seems to complain about `strcmp` function not available in
the vfork patch (https://github.com/llvm/llvm-project/pull/81564):

https://lab.llvm.org/buildbot/#/builders/68/builds/70093/steps/6/logs/stdio



Unfortunately, I can't reproduce the failure on my linux machine so this
is a guessing fix. If anyone has a way to reproduce and very this fix,
please feel free to merge this change.

Co-authored-by: jeffreytan81 <jeffreytan@fb.com>