spoolss: Fix caching of printername->sharename

time_mono() gives seconds since boot, gencache expects seconds since epoch.
With time_mono(), the values are always expired immediately.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12374

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>

Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Thu Oct 13 13:48:52 CEST 2016 on sn-devel-144
