Bug#963520: guitarix: stop using rename.ul in d/rules

[Chris Hofstaedtler]

Source: guitarix
Version: 0.39.0+dfsg1-3.1

Dear Maintainer,

your package appears to use `rename.ul` from util-linux, which will
stop installing this binary soon. Please use `rename` from prename
or other tools instead.

Thanks,
Chris