xb-silo: Cancel the GFileMonitor before unreffing it

GFileMonitors currently need to be explicitly cancelled when they are
unreffed, otherwise they can deadlock due to the GLib-internal worker
thread also holding a strong reference to the monitor.

For full details, see: https://gitlab.gnome.org/GNOME/glib/-/issues/1941

Closes #59

src/xb-silo.c

@@ -1349,6 +1349,7 @@ xb_silo_machine_fixup_attr_text_cb (XbMachine *self,
 static void
 xb_silo_file_monitor_item_free (XbSiloFileMonitorItem *item)
 {
+	g_file_monitor_cancel (item->file_monitor);
 	g_signal_handler_disconnect (item->file_monitor, item->file_monitor_id);
 	g_object_unref (item->file_monitor);
 	g_slice_free (XbSiloFileMonitorItem, item);