gcal-timeline: completed_calendars should be unsigned

In on_calendar_monitor_completed_cb() we have this assert:

g_assert (self->completed_calendars <= g_hash_table_size (self->calendars));

That is comparing a signed integer to unsigned. This is dangerous. We
could add casts to say that we really want to do it, but
completed_calendars should never be negative, so might as well use
unsigned.

src/core/gcal-timeline.c

@@ -70,7 +70,7 @@ struct _GcalTimeline
   gchar              *filter;
 
   GHashTable         *calendars; /* GcalCalendar* -> GcalCalendarMonitor* */
-  gint                completed_calendars;
+  guint               completed_calendars;
 
   GHashTable         *subscribers; /* GcalTimelineSubscriber* -> SubscriberData* */
   GcalRangeTree      *subscriber_ranges;