weston-log: Add 'destroy_subscription' callback for the subscription

As 'new_subscription' can create additional objects, 'destroy_subscription'
will be needed when cleaning up.

As this requires a libweston_major bump (noticed by @pq), bump it up to
8.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Daniel Stone <daniel.stone@collabora.com>

pipewire/pipewire-plugin.c

@@ -819,7 +819,7 @@ weston_module_init(struct weston_compositor *compositor)
 			compositor->weston_log_ctx,
 			"pipewire",
 			"Debug messages from pipewire plugin\n",
-			NULL, NULL);
+			NULL, NULL, NULL);
 
 	pipewire->destroy_listener.notify = weston_pipewire_destroy;
 	wl_signal_add(&compositor->destroy_signal, &pipewire->destroy_listener);