Bug 37733: Fix header menu style for preservation link

Bug 37733: Fix header menu style for preservation link

This patch adds the missing "dropdown-item" class to the link in the
header menu for the preservation module.

To test, apply the patch and enable the preservation module
(PreservationModule).

Reload the page and click "More" in the header menu. The "Preservation"
link should be styled like all the others.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>