Explicitly include <sched.h> for sched_yield()

It might be implicitly included by <pthread.h> (and that's why it worked
so far), but it's not guaranteed (for example, this is not the case with
newlib).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223661 91177308-0d34-0410-b5e6-96231b3b80d8