Fix time sync service

It is expected to not output anything on stdout. Especially remote end
may be already terminated, so writing there would result in EPIPE.

Fixes QubesOS/qubes-issues#1592

qubes-rpc/qubes.SetDateTime

@@ -8,4 +8,4 @@ if [ $timediff -le 2 -a $timediff -ge -2 ]; then
     # don't bother
     exit 0
 fi
-date -u -s "$timestamp"
+date -u -s "$timestamp" >/dev/null