Fix build on non-Windows platforms.

llvm-svn: 211288

llvm/include/llvm/Support/Mutex.h

@@ -15,11 +15,13 @@
 #define LLVM_SUPPORT_MUTEX_H
 
 #include "llvm/Support/Compiler.h"
-#include "llvm/Support/Threading.h"
 #include <cassert>
 
 namespace llvm
 {
+  // Forward declare this function.
+  bool llvm_is_multithreaded();
+
   namespace sys
   {
     /// @brief Platform agnostic Mutex class.