Fix: rculfhash only needs to include urcu-pointers.h

rculfhash is not meant to use a specific urcu flavor, but rather receive
the flavor to use as parameter to _cds_lfht_new().

All we need is the API of urcu-pointers.h, so include that instead.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>