calendar-server: Add back missing return value

In commit 7e0e224e0, when moving from e_cal_recur_generate_instances()
to e_cal_client_generate_instances(), the return value of the
ECalRecurInstanceFn callback was accidentally removed; add it
back.

https://bugzilla.gnome.org/show_bug.cgi?id=769156

src/calendar-server/gnome-shell-calendar-server.c

@@ -443,6 +443,8 @@ generate_instances_cb (ECalComponent *comp,
   occurrence->rid        = e_cal_component_get_recurid_as_string (comp);
 
   appointment->occurrences = g_slist_append (appointment->occurrences, occurrence);
+
+  return TRUE;
 }