LP#1378575: teach action::hold_request about behind_desk

This patch teaches the CDBI class for the action.hold_request
table about the newish behind_desk column, preventing errors
like this from being thrown if a recent OpenSRF version is in
use:

Processing of hold failed:  Can't locate object method "behind_desk"
via package "action::hold_request" at /usr/local/share/perl/5.14.2/OpenILS/Application/Storage/CDBI.pm line 181.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>