Don't output diplicate classes to rlibmap-generated rootmap

Duplicates could show in at least two ways:

(1) duplicate lines in LinkDef file
(2) same LinkDef file given a number of times

This minimal patch should solve both cases even if the same file is added
multiple times under different paths.

Signed-off-by: Benjamin Bannier <bbannier@bnl.gov>
Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>