Fixed build.bat so it works correctly under paths containing a space

I tried to run build.bat today and it stopped immediately with an MSBuild error saying something about only being able to build one project at a time. I realized `build.bat` wasn't wrapping the filename in quotes before passing it to `msbuild`, which was being tripped up because the path contained a folder with spaces in the name.

I fixed `build.bat` so it quotes the filenames before passing them to `msbuild`, and it now it's able to run as expected.