kcm_clock helper: Sync the hwclock after setting the date.

Recent versions of systemd no longer do this automatically on shutdown, so if
we don't take care of writing to the hardware clock ourselves, the time will be
reset after a reboot.

https://bugzilla.redhat.com/show_bug.cgi?id=749516

Approved by ossi.
REVIEW: 102985
FIXED-IN: 4.7.4

(cherry picked from commit 240c655e)