wfcqueue: add C++ compatibility API

Introduce __cds_wfcq_head_cast and cds_wfcq_head_cast for compability
of wfcqueue with c++. Those are effect-less in C, where transparent
unions are supported. However, in C++, those transform struct
cds_wfcq_head and struct __cds_wfcq_head pointers to
cds_wfcq_head_ptr_t.

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>