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>

xwayland/launcher.c

@@ -398,7 +398,7 @@ weston_module_init(struct weston_compositor *compositor)
 		weston_compositor_add_log_scope(wxs->compositor->weston_log_ctx,
 						"xwm-wm-x11",
 						"XWM's window management X11 events\n",
-						NULL, NULL);
+						NULL, NULL, NULL);
 
 	return 0;
 }