Don't write #include "Windows/WindowsSupport.h" from the Windows dir.

This generates -Wnonportable-include-dir warnings, and doesn't need
to be there.  It seems this was just checked in on accident.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350655 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/Windows/Threading.inc b/lib/Support/Windows/Threading.inc
index decb488..0bd92f6 100644
--- a/lib/Support/Windows/Threading.inc
+++ b/lib/Support/Windows/Threading.inc
@@ -14,7 +14,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/Twine.h"
 
-#include "Windows/WindowsSupport.h"
+#include "WindowsSupport.h"
 #include <process.h>
 
 // Windows will at times define MemoryFence.