Fix systemd failure with lock files

Systemd introduced a new policy described here:
http://lists.freedesktop.org/archives/systemd-devel/2011-March/001823.html

Add the "lock" group to buildroot to allow systemd to set this in place
at boot time

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

fs/skeleton/etc/group

@@ -9,6 +9,7 @@ wheel:x:10:root
 audio:x:29:
 utmp:x:43:
 staff:x:50:
+lock:x:54:
 haldaemon:x:68:
 dbus:x:81:
 netdev:x:82: