Bug 29961: Make horizontal scroll bar always visible on acqui/z3950_search.pl

Bug 29961: Make horizontal scroll bar always visible on acqui/z3950_search.pl

To test:
1. In Acquisitions make a basket and add something "From an external source"
2. Search for a title and click on the 'MARC' button, make sure the MARC prview is both tall and wide.
3. You will not see the horizontal scroll bar unless you scroll all the way to the bottom of the preview.
4. Apply patch.
5. Try steps 1 & 2 again, this time you should see the horizontal scroll bar as soon as the MARC prveiw loads.

Note: I put the CSS on multiple lines for better readability

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>