Add a preconfigured gio/gnetworking.h for Windows

Since Windows builds by Visual C++ do not make use of autotools during
its build process, we need to dist a pre-configured
gio/gnetworking.h(.win32) for such builds.

The vs9/vs10 (and therefore vs11) property sheets are updated as well

https://bugzilla.gnome.org/show_bug.cgi?id=690163