plugins: rtc: follow-up to bc8118d, stop timer at shutdown

Introduced to work-around systems with broken RTCs, it also introduced a
dormant bug in Finit, triggered at shutdown on some systems.  The call
to dlclose() removed the memory to the rtc_timer which libuev later then
referenced.

Fixes #429

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

plugins/rtc.c

@@ -338,6 +338,7 @@ PLUGIN_INIT(plugin_init)
 
 PLUGIN_EXIT(plugin_exit)
 {
+	uev_timer_stop(&rtc_timer);
 	plugin_unregister(&plugin);
 }