[libc] add missing header dependencies for sched objects (#78741)

This patch fixes full build problems in
https://github.com/llvm/llvm-project/issues/78721 (the header problem).
The `libc.a` target can be built now.

As a separate issue, `check-libc` is failing because undefined symbols
from `libunwind`, which I do not actually know the reason yet. I will be
looking into it.