Bug 34289: Add missing class to one condition of the checkin template

Bug 34289: Add missing class to one condition of the checkin template

The markup of the checkin template varies depending on the conditions of
the operation, and in one case the barcode field was missing the
"barcode" class. This patch adds it.

To test, apply the patch and make sure the finesMode system preference
is enabled.

- Go to returns.pl (/cgi-bin/koha/circ/returns.pl)
- Expand the the options so you see 'Forgive overdue charges'.
- Check that box and check in an item
- After checkin, the barcode field should keep the same style it had
  before the transaction.
- Note: The yellow background of the input when the "Forgive" checkbox
  is checked is to be expected.

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>