Make Win32's header file name lower for cross build on case-sensitive filesystem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124864 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/unittest/UnitTestMain/TestMain.cpp b/utils/unittest/UnitTestMain/TestMain.cpp
index 4469c03..b35bae5 100644
--- a/utils/unittest/UnitTestMain/TestMain.cpp
+++ b/utils/unittest/UnitTestMain/TestMain.cpp
@@ -13,7 +13,7 @@
 
 
 #if defined(LLVM_ON_WIN32)
-# include <Windows.h>
+# include <windows.h>
 # if defined(_MSC_VER)
 #   include <crtdbg.h>
 # endif