[libc] Add `errno_h` as a dependency to `sigaddset` and `sigemptyset`.

Summary: The bots are catching this missing dependency.

Reviewers: PaulkaToast, abrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D75502