Bug 14660: Fix stocknumberam123.pl cataloguing plugin

Bug 14660: Fix stocknumberam123.pl cataloguing plugin

The cataloguing plugin stocknumberam123.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
 - Create a new item, set the inventory number to: A 0000000002
 - Start to catalog another item, enter A into inventory and
 - Enter A 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>