Do not include pthread.h and sched.h when threads are disabled

Patch by Philippe Daouadi!

http://reviews.llvm.org/D9639


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@246168 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed