libcxx: fix bootstrapping for mingw-w64

Differential Revision: https://reviews.llvm.org/D33388

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@303928 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__threading_support b/include/__threading_support
index 385fff3..afd7cb5 100644
--- a/include/__threading_support
+++ b/include/__threading_support
@@ -27,7 +27,7 @@
 # include <pthread.h>
 # include <sched.h>
 #elif defined(_LIBCPP_HAS_THREAD_API_WIN32)
-#include <Windows.h>
+#include <windows.h>
 #include <process.h>
 #include <fibersapi.h>
 #include <__undef_min_max>