Unable to build MvvmCross_Mac.sln on Mac (at least with with XS 4.2.3).

This pull request fixes small details that prevented MvvmCross_Mac.sln to build:
- "The type or namespace name 'MonoMac' could not be found" error in *.Mac library projects
- "The type or namespace name 'Cirrious' could not be found" error in *.Mac executable projects