System/Windows: Reduce dependencies.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118504 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/System/Win32/Win32.h b/lib/System/Win32/Win32.h
index 55ab3e7..d054812 100644
--- a/lib/System/Win32/Win32.h
+++ b/lib/System/Win32/Win32.h
@@ -18,6 +18,7 @@
 
 // Require at least Windows 2000 API.
 #define _WIN32_WINNT 0x0500
+#define WIN32_LEAN_AND_MEAN
 
 #include "llvm/Config/config.h" // Get build system configuration settings
 #include <Windows.h>