Bug 7843: Followup - fix broken url

Bug 7843: Followup - fix broken url

This patch fixes an invalid URL when clicking
on RSS feed.

To test:
1) Using Nicole words :)
If you visit : http://mykoha/cgi-bin/koha/opac-main.pl
then the RSS feed works.
If you're on http://mykoha and you click the rss icon you
get : http://mykoha/opac-news-rss.pl which is a 404 page.

2) Apply the patch

3) Repeat 1, error is now fixed

Signed-off-by: Pierre <tredok.pierre@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>