Update from private pointer to new private methods
Varnish 6.6 updates the interface for private pointers in vmods [1]:
The free pointer in struct vmod_priv has been replaced with a
pointer to struct vmod_priv_methods, to where the pointer to the
former free callback has been moved as the fini member.
Bug: T378737
[1] https://varnish-cache.org/docs/6.6/whats-new/changes-6.6.html#private-pointers