Prevent Protobuild synchronization from occurring

Currently the MonoGame definition file is manually maintained (it has comments and grouping etc.), which means any Protobuild synchronisation with change the entire file (and it means Git will mark the repository as dirty, which is annoying as a Git sbumodule).

This prevents Protobuild from synchronizing C# projects back to the MonoGame definition under any circumstances, particularly:

  * When --resync or --sync is used on the command line
  * When Protobuild is being invoked with MonoGame as a submodule (where the parent module is resyncing)

The changes made in Protobuild.exe are in this commit: https://github.com/hach-que/Protobuild/commit/e4852cbb1f9c00f7eace1cf4f71f62c5c4657858