BacklinkCache was forgetting to check rd_interwiki when getting redirects, and thus...

BacklinkCache was forgetting to check rd_interwiki when getting redirects, and thus getting
some extranous backlinks for the redirect table (if there happened to be a page redirecting
to an interwiki page with the same name plus an interwiki prefix).

Not a major issue since this is really only used as far as i can tell, to purge squids and html file cache.
A couple of extra unneeded purges shouldn't hurt anything, just a bit pointless.