Don't build io_win32 for Xbox One

This file contains a lot of code that doesn't compile for Xbox One and doesn't appear to be necessary for usage on that platform. _WIN32 is defined for the Xbox One platform.

src/google/protobuf/io/io_win32.cc

@@ -42,7 +42,7 @@
 //
 // This file is only used on Windows, it's empty on other platforms.
 
-#if defined(_WIN32)
+#if defined(_WIN32) && !defined(_XBOX_ONE)
 
 // Comment this out to fall back to using the ANSI versions (open, mkdir, ...)
 // instead of the Unicode ones (_wopen, _wmkdir, ...). Doing so can be useful to