[cmake] simplify include dirs handling in ROOT_GENERATE_DICTIONARY
[cmake] simplify include dirs handling in ROOT_GENERATE_DICTIONARY

Keep list with plain directories names, 
only for rootcling executable provide list with prefixed -I
olifre/root,olifre/root,olifre/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,karies/root