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>

libweston/backend-drm/drm.c

@@ -3408,7 +3408,7 @@ drm_backend_create(struct weston_compositor *compositor,
 	b->debug = weston_compositor_add_log_scope(compositor->weston_log_ctx,
 						   "drm-backend",
 						   "Debug messages from DRM/KMS backend\n",
-						    NULL, NULL);
+						    NULL, NULL, NULL);
 
 	compositor->backend = &b->base;