Add some Visual Studio gyp artifacts to .gitignore

These three msbuild-related files are generated by gyp on Windows.

I considered a more general .gitignore change that would catch other cases of the same types of files, but it wasn't clear to me how to scope it. For example, *.xml turns up quite a few files (mostly test files, I think).

My current approach is to target these three files very precisely for now, and figure out how to generalize if other examples arise.

/cc @piscisaureus