NFC (build fix): Add header for llvm::errs().

Looks like e9211e0393 unfortunately broke the sanitizer build bots,
because those bots compile the symbolizer with DLLVM_ENABLE_THREADS=Off.
Likely, before the patch, this header was transitively included.

llvm/lib/Support/ThreadPool.cpp

@@ -14,6 +14,7 @@
 
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Threading.h"
+#include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;