Workaround for QSBR mode issue #4390

[Robert Kirkman]

Hello, we have developed this workaround for the hanging problem with builds of bind with the configure argument --with-liburcu=qsbr :

the hanging problem is reproducible on both ARM Android and ARM GNU/Linux, and is fixed on both by this.

```
--- a/lib/isc/work.c
+++ b/lib/isc/work.c
@@ -276,6 +276,7 @@ workthread_thread(void *arg) {
 		work_run(work);
 	}

+	rcu_thread_offline();
 	isc__loopmgr_stopping(thread->loop);

 	return NULL;
```

We hope this helps someone.

https://gitlab.isc.org/isc-projects/bind9/-/work_items/4390

https://github.com/termux/termux-packages/issues/31302