Add Xamarin.Mac to supported framework files for PCLs

@jstedfast added the new Xamarin.iOS unified framework to the PCL profile xml files in another [commit](https://github.com/mono/bockbuild/commit/f2381976152a729728211adbbed61562bf887dec).

This pull request includes a change that adds the new Xamarin.Mac unified framework to the PCL profile xml files. This allows Xamarin Studio 5.4 to add the Json.NET NuGet package to a Xamarin.Mac unified project.