[llvm] [CAS] Fix AIX build (PR #159647)

[David Tenty via llvm-commits]

https://github.com/daltenty approved this pull request.

Change LGTM, flock requires libbsd on AIX:
https://www.ibm.com/docs/en/aix/7.1.0?topic=l-lockfx-lockf-flock-lockf64-subroutine

Thanks!

https://github.com/llvm/llvm-project/pull/159647