idle-monitor: Reset the alarm so it can be triggered again

http://www.x.org/archive/X11R7.6/doc/xextproto/sync.txt says:

[... If] the delta is 0 and test-type is PositiveComparison
or NegativeComparison, no change is made to value and the alarm
state is changed to Inactive before the event is generated.

But we want to receive to receive events about the idleness/business
of the counter, so reset the alarm when the alarm is triggered.

https://bugzilla.gnome.org/show_bug.cgi?id=688227