nginx-http-auth: match when "referrer" is present

I think this was broken by ac061155f093464fb6cd2329d3d513b15c68e256 which introduced the anchor at the end of the line, so that log lines with a "referrer" at the end would no longer match.

Please note that I'm fairly inexperienced with regexes, and very inexperienced with defensively designing them - I'd be grateful if you give my change an extra look, to be sure I'm not introducing any vulnerabilities.