subprocess: Fix cross-compiling with mingw toolchain

Github-Pull: arun11299/cpp-subprocess#99
Rebased-From: 34033d03deacfdba892a708b7d8092b4d9e5e889

Author: hebasto

src/util/subprocess.h

@@ -65,7 +65,7 @@ Documentation for C++ subprocessing library.
 
 extern "C" {
 #ifdef __USING_WINDOWS__
-  #include <Windows.h>
+  #include <windows.h>
   #include <io.h>
   #include <cwchar>