Add files generated by "make depend", PGO and gcov to .gitignore

[Peter Davies]

> Is best practice to confine these to a particular folder (like you get
> with -fprofile-dir)?  Is there a switch to do that for gcov data?

Due to limitations of my patch -fprofile-dir (or passing arguments to
-fprofile-* is unsupported). All it allows you can do is pass
"--coverage", "-fprofile-generate" or "-fprofile-use" to winegcc
without arguments. It started as a hack to enable gcov and evolved
into a hack to enable PGO and gcov.

Peter