Small fix in handling of "unfree" meta.license attributes

meta.license is can be a string or a list of strings. But there is one
unhandled case where "unfree" (or "unfree-redistributable") is a part of
a list. It will currently not be detected as an "unfree" package and
Hydra will attempt to build it. This should fix it.

Example: http://hydra.nixos.org/build/6553461