Fix MSVC build with LLVM_ENABLE_THREADS=OFF

Follow-up to the ThreadPool implementation.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255621 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed