context: make driver runnable when we add a runnable node

Explicitly make the driver runnable when we add a runnable node to it.

We have to do this because the runnable node might not be directly linked
to this driver.

In the case of the echo-canceler, for example, a playback stream
can be linked to the sink before the echo canceled virtual sink.
All echo-cancel nodes are added to a source driver, which would not be
activated because there is no direct path from the playback stream
to the source river.

Fixes #3145

src/pipewire/context.c

@@ -943,6 +943,8 @@ static void move_to_driver(struct pw_context *context, struct spa_list *nodes,
 	spa_list_consume(n, nodes, sort_link) {
 		spa_list_remove(&n->sort_link);
 
+		driver->runnable |= n->runnable;
+
 		pw_log_debug(" follower: %p %s runnable:%u driver-runnable:%u", n, n->name,
 				n->runnable, driver->runnable);
 		pw_impl_node_set_driver(n, driver);