Avoid clobbering of ROOT dictionaries.

As noted by @bendavid and after checking that indeed it is in the CMSSW:
https://github.com/cms-sw/cmssw-config/blob/master/SCRAM/Plugins/BuildRules.pm#L1190

No problems found after recompiling like this and loading `libHiggsAnalysisCombinedLimit` in ROOT.