Include Threading.h instead of forward declaring a function.

Previously this led to a circular header dependency, but a recent
change has since removed this dependency, so the correct fix is
to simply include the header rather than forward declare.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211311 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Mutex.h

@@ -15,13 +15,11 @@
 #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.