xen/timer: re-introduce the inittodr call in the resume path

r298930 removed the inittodr call, but it seems like this prevents
"calcru: runtime went backwards ..." messages from occasionally appearing
when resuming from migration.

Reported by:	Karl Pielorz <kpielorz@tdx.co.uk>
Sponsored by:	Citrix Systems R&D

sys/dev/xen/timer/timer.c

@@ -477,6 +477,9 @@ xentimer_resume(device_t dev)
 	/* Reset the last uptime value */
 	pvclock_resume();
 
+	/* Reset the RTC clock */
+	inittodr(time_second);
+
 	/* Kick the timers on all CPUs */
 	smp_rendezvous(NULL, xentimer_percpu_resume, NULL, dev);