time-selector: fix AM to PM time logic

Currently, the hour component of the time is doubled when switching from
AM to PM, which is incorrect. This commit fixes the logic by adding 12
hours when switching to PM.

https://bugzilla.gnome.org/show_bug.cgi?id=765370

src/gcal-time-selector.c

@@ -106,7 +106,7 @@ update_time (GcalTimeSelector *selector)
       g_signal_handlers_block_by_func (selector->period_combo, update_time, selector);
 
       gtk_combo_box_set_active (GTK_COMBO_BOX (selector->period_combo), hour > 12);
-      hour *= 2;
+      hour += 12;
 
       g_signal_handlers_unblock_by_func (selector->period_combo, update_time, selector);
     }