audio/deadbeef: enable both MP3 decoding backends by default

Historically, libmad was the first choice, as there probably were no
libmpg123 at the time, only the console player (mpg123), and because
it provided best precision out of integer math*. Later several bugs
had been found in libmad which resulted in the incorrect decoding of
some MP3 files, so libmpg123 support was added and made the default,
and MAD backend was left for flexibility and testing. By that time,
there should not be significant performance differences between both
backends on commodity hardware.

Upstream enables both of them in the official binaries for GNU/Linux
and macOS, they can be switched on the fly without rebuilding, so we
should follow the same path (offering more featureful package) which
can become handy if one backend suddenly breaks, e.g. see PR 264442.

*) https://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg1201101.html