Skip citation label warnings on Sphinx [source] links.

Sphinx [source] links generated citation label warnings "could not
relabel citation reference [source]" which didn't make sense.  This
patch overrides the generation of a warning for that specific case.

It might make more sense to add a set attribute to the
``app.bibtex_cache`` to track labels that have already generated
warnings so that warnings get produced only one time per label.  This
solves the immediate problem.