Bug 32114: Add missing html_helpers.inc

Bug 32114: Add missing html_helpers.inc

This patch makes a minor modification to the OPAC
search results RSS template so that an error isn't
triggered. The html_helpers include was missing.

To test, apply the patch and perform a catalog
search in the OPAC which will return results.

At the end of the "Your search results" heading
you should see an orange RSS icon. Clicking on it
should take you to an RSS feed instead of an
error page.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>