target skeleton: add audio group

Add audio group to the target skeleton.
Some multimedia applications based on alsa-lib need it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

fs/skeleton/etc/group

@@ -6,6 +6,7 @@ adm:x:4:
 tty:x:5:
 disk:x:6:
 wheel:x:10:root
+audio:x:29:
 utmp:x:43:
 staff:x:50:
 haldaemon:x:68: