Bug 5839: Add index stocknumber on 952$i

Bug 5839: Add index stocknumber on 952$i

This patch adds a new index for stocknumber on field 952$i.

Note: For testing you have to copy over the changed files
from  kohaclone/etc/zebradb/ to your koha-dev/etc/zebradb folders.
Reindex.

To test:
1) Add 952$i to your frameworks
2) Add an item with 952$i
3) Search for your 952$i value in keyword search
4) Search for stocknumber, using stocknumber:<your 952$i value> or inv:<...

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>