Fix Content Pipeline build errors on OS X

While trying to get the MGCB tool to compile on OS X, I saw that the `MonoGame.Framework.Content.Pipeline.MacOS` project was giving out errors. After some spelunking, I found that some DLL's and class files were left out of the project. This fixes that.

I don't have a Linux distro setup to test right now, but I believe the same issue carries over with their equivalent project as well.