mpg123: add PACKAGECONFIG for pulseaudio and alsa

The default DISTRO_FEATURES include alsa and pulseaudio at the same
time. Hence, both of the options are enabled in mpg123 configuration
without adding related dependencies, which causes build error.

Make the options mutually exclusive through PACKAGECONFIG.  If both alsa
and pulseaudio are specified, pulseaudio takes precedence.

Signed-off-by: Eren Türkay <eren@hambedded.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>