Fix Windows Phone 8 compilation when using VS2013.2.

You just can't compile WP8 version when using VS2013.2.
https://connect.microsoft.com/VisualStudio/feedback/details/865927/compiling-monogame-for-windowsphone-fails
. A simple full name reference fixes the problem. This is a temporary
fix, but with no down side. This probably won't break any other version.