pulseaudio: Add audio group explicitly

pulseaudio: Add audio group explicitly

Since pulseaudio-server requires the audio group, we explicitly add it.

When use useradd-staticids or do not use the default group in
base-passwd, an error will occur because the audio group is not defined.

NOTE: pulseaudio: Performing useradd with [--root
TOPDIR/tmp/work/cortexa72-poky-linux/pulseaudio/17.0/recipe-sysroot
--home-dir /var/run/pulse --gid 998 --groups audio,pulse
--no-create-home --system --shell /bin/false --uid 998 pulse]
useradd: group 'audio' does not exist
ERROR: pulseaudio: useradd command did not succeed.

Signed-off-by: Kyungjik Min <dpmin7@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>