Fix missing urcu_ref.h in Makefile.am

Fix missing urcu_ref.h in Makefile.am

The 0.4.7 tarball is missing 'urcu/urcu_ref.h', causing the following
error:

      CC     rculfqueue.lo
    In file included from rculfqueue.c:26:
    urcu/rculfqueue.h:26:22: error: urcu_ref.h: No such file or directory
    In file included from rculfqueue.c:26:
    urcu/rculfqueue.h:45: error: field ‘ref’ has incomplete type
    In file included from rculfqueue.c:27:
    urcu/rculfqueue-static.h:29:27: error: urcu/urcu_ref.h: No such file or directory

Reported-by: Jon Bernard <jbernard@debian.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>