Bug 18573: (bug 17847 follow-up) Update av-build-dropbox.inc for OPAC

Bug 18573: (bug 17847 follow-up) Update av-build-dropbox.inc for OPAC

On bug 17847, av-build-dropbox.inc has been updated for the intranet,
not OPAC

There is only one call to av-build-dropbox.inc at the OPAC, from
opac-suggestion.tt

Test plan:
Create a new suggestion, anonymous or with a logged in user
=> Without this patch you got
  Template process failed: undef error - The method default is not
  covered by tests! at /home/vagrant/kohaclone/C4/Templates.pm line 121.
=> With this patch applied you should see the item type dropdown list
correctly filled

Reproduced without patch, OK with patch
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>