Fix conditional include in ThreadPool

Should fix  https://lab.llvm.org/buildbot#builders/37/builds/10259

Author: serge-sans-paille

llvm/lib/Support/ThreadPool.cpp

@@ -13,7 +13,12 @@
 #include "llvm/Support/ThreadPool.h"
 
 #include "llvm/Config/llvm-config.h"
+
+#if LLVM_ENABLE_THREADS
 #include "llvm/Support/Threading.h"
+#else
+#include "llvm/Support/raw_ostream.h"
+#endif
 
 using namespace llvm;