libceph: advertise support for OSD_POOLRESEND

The code has been in place since commit 63244fa1

 ("libceph:
introduce ceph_osd_request_target, calc_target()"), and, with the
ceph_{oloc,oid}_copy() issue fixed in the previous commit, is now
in working order.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>