use a relative symbolic link

This triggers the Lintian warning [source-contains-unsafe-symlink](https://lintian.debian.org/tags/source-contains-unsafe-symlink.html). However, since that symlink will just be copied into the final binary package, this error is kind of wrong. I'd still replace it with a relative link, I wouldn't know a reason to keep the absolute path inside the link. Otherwise just close this PR and add a Lintian override for this tag.