Add missing -lwfqueue to add rcu flavors

Otherwise, linking with the flavor .so leads to:

/usr/local/lib/liburcu-bp.so: undefined reference to `cds_wfq_enqueue'
/usr/local/lib/liburcu-bp.so: undefined reference to `cds_wfq_init'
/usr/local/lib/liburcu-bp.so: undefined reference to `cds_wfq_node_init'

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