Fixes high CPU usage on timer expiration when using glib event loop.

GTimerSource has 1ms accuracy, QTimerInfoList::timerWait() has 1us accuracy.
So when there is less than 1 ms left on the timer we enter a tight loop
checking for timer expiration since we don't round up the timerWait()
timer to the nearest millisecond.

Task-number: QTBUG-7618
Change-Id: I684c9236324f598bc69c6810be270aa47c791f91
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>