Bug 14660: Fix stocknumberAV.pl cataloguing plugin

Bug 14660: Fix stocknumberAV.pl cataloguing plugin

The cataloguing plugin stocknumberAV.pl is broken and no
longer generating numbers - this patch fixes it.

To test:
- Link a subfield to the plugin - usually it's 952$i or the UNIMARC
  equivalent in your MARC frameworks
- Set up a new authorised value INVENTORY, the code is an uppercase
  prefix - "B"
- The Descriptions is your starting number - 1
- The OPAC description remains empty
- Catalalog an item
- Enter B as stocknumber and activate the plugin by clicking on
  ... at the end of the field
- The number will not get added - Firebug shows an error:
  Undefined subroutine CGI::output_html_with_http_headers
- Apply patch
- Verify numbers are now generated and no errors are shown

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>