Bug 20951: add missing use Koha::Patron::Debarments in Koha::Patron::Discharge

Bug 20951: add missing use Koha::Patron::Debarments in Koha::Patron::Discharge

Koha::Patron::Discharge was missing use Koha::Patron::Debarments used in discharge() method.

Test plan :
1) Enable pref useDischarge
2) Go to OPAC user page
3) Go to 'ask for a discharge' tab
4) Click on 'Ask for a discharge'
5) Go to staff interface : /cgi-bin/koha/members/discharges.pl
6) Click on Allow
=> Without patch you get software error
=> With patch the discharge is validated

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>