Modify '.gitignore' to ignore Visual Studio files.

DOOM-3 builds and is edited with Visual Studio. However a large number
of files generated by the program are preference files or used for
IntelliSense. These hold little weight inside a repository.

Also; normally the build path is '[Rr]elease/' and '[Dd]ebug/' but
DOOM-3 moves all builds into the 'build/'. Thus by ignoring '[Bb]uild/'
all output files are of no concern of Git's. The repository already
ignored build files but '[Bb]uild/' is more robust.