private-etc: add pipewire dir to @Sound group

The `@sound` group currently only contains paths related to alsa and
pulseaudio.

This amends commit 02d3768 ("private-etc rework: file groups moved to
src/include/etc_groups.h, new groups added", 2023-01-25).

Relates to #5610 #6400.

src/include/etc_groups.h

@@ -70,6 +70,7 @@ static char *etc_group_sound[] = {
 	"alsa",
 	"asound.conf",
 	"machine-id", // required by PulseAudio
+	"pipewire",
 	"pulse",
 	NULL
 };