pkgconfig: Export corosysconfdir

Useful for external code to easily tell where corosync.conf
is (in case someone configured it for /usr/local/etc, ...)

E.g. pacemaker's crm_report collects corosync.conf, and some
of its testing tools generate a corosync.conf for a test cluster.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>