system/skeleton: add mail group

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

system/skeleton/etc/group

@@ -6,6 +6,7 @@ adm:x:4:
 tty:x:5:
 disk:x:6:
 lp:x:7:
+mail:x:8:
 kmem:x:9:
 wheel:x:10:root
 cdrom:x:11: