Speculative build fix for lld on Linux after Michael's #include removals

llvm-svn: 320645
diff --git a/llvm/lib/Support/ThreadPool.cpp b/llvm/lib/Support/ThreadPool.cpp
index a5dc6b5..d0212ca 100644
--- a/llvm/lib/Support/ThreadPool.cpp
+++ b/llvm/lib/Support/ThreadPool.cpp
@@ -15,6 +15,7 @@
 
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Threading.h"
+#include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;