The summary says it all also see bug 185215 .
Just a minor doubt whether it should be called "zlib License" or "zlib/libpng License" (like in http://www.opensource.org/licenses/). I think I'll go with the former, will leave open for other opinions in case there are some before the next rpmlint package revision.
I choose "zlib License" because that is what is in the upstream rpmlint files license list (which our package overrides).
Done in 0.76-1.