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.

(From OE-Core rev: 4fc918da4667eebbbdae3def8c38209a3d650f97)

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

meta-selftest/files/static-group

@@ -28,3 +28,4 @@ sgx:x:528:
 ptest:x:529:
 xuser:x:530:
 seat:x:531:
+audio:x:532:

meta/recipes-multimedia/pulseaudio/pulseaudio.inc

@@ -147,7 +147,7 @@ do_install:append() {
 }
 
 USERADD_PACKAGES = "pulseaudio-server"
-GROUPADD_PARAM:pulseaudio-server = "--system pulse"
+GROUPADD_PARAM:pulseaudio-server = "--system audio; --system pulse"
 USERADD_PARAM:pulseaudio-server = "--system --home /var/run/pulse \
                               --no-create-home --shell /bin/false \
                               --groups audio,pulse --gid pulse pulse"