skeleton: add "operator" group

gid 37 was referenced in /etc/passwd but not in /etc/group

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

system/skeleton/etc/group

@@ -16,6 +16,7 @@ video:x:28:
 audio:x:29:
 tape:x:32:
 www-data:x:33:
+operator:x:37:
 utmp:x:43:
 plugdev:x:46:
 staff:x:50: