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>

include/libweston/weston-log.h

@@ -67,6 +67,7 @@ weston_compositor_add_log_scope(struct weston_log_context *compositor,
 				  const char *name,
 				  const char *description,
 				  weston_log_scope_cb new_subscription,
+				  weston_log_scope_cb destroy_subscription,
 				  void *user_data);
 
 void