Fix: use liburcu flavor header instead of local copy

The layout of struct rcu_flavor_struct has changed in newer URCU
versions. There is no need to keep a local copy of this structure. Use
the library header instead.

This ensures that the caller of _cds_lfht_new(), which passes a flavor
pointer with the library layout defined by liburcu, has the same layout
expected internally within the local copy of rculfhash.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>