Bug 8294: Fix display of serial enumeration for OPAC holds

Bug 8294: Fix display of serial enumeration for OPAC holds

It turns out that this was not a missing but a broken feature.
This patch corrects the template variables to make enumchron
show up on the item table for item level holds in the OPAC.

To test:
Try to place item level holds on
1) a record where at least one item
2) a record where none of the items
has items.enumchron information.

The column with enumchron should only show for 1), but not
for 2).

Signed-off-by: David Cook <dcook@prosentient.com.au>
Works as described.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>