trigger: add filter in trigger monitor buffer

The filter can be set with /trigger monitor <filter> or by entering data
in the trigger monitor input.

src/plugins/trigger/trigger.c

@@ -1218,6 +1218,7 @@ weechat_plugin_end (struct t_weechat_plugin *plugin)
     /* make C compiler happy */
     (void) plugin;
 
+    trigger_buffer_end ();
     trigger_config_write ();
     trigger_free_all ();
     trigger_config_free ();