Prevent iovec redefinition on C++.

Author: redboltz

include/msgpack/v1/vrefbuffer.hpp

@@ -1,7 +1,7 @@
 //
 // MessagePack for C++ zero-copy buffer implementation
 //
-// Copyright (C) 2008-2013 FURUHASHI Sadayuki and KONDO Takatoshi
+// Copyright (C) 2008-2017 FURUHASHI Sadayuki and KONDO Takatoshi
 //
 //    Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
@@ -22,7 +22,7 @@
 #endif
 #endif // defined(_MSC_VER)
 
-#ifndef _WIN32
+#if defined(unix) || defined(__unix) || defined(__APPLE__) || defined(__OpenBSD__)
 #include <sys/uio.h>
 #else
 struct iovec {