FWCore/PluginManager: gCINTMutex -> gInterpreterMutex

gCINTMutex was renamed to gInterpreterMutex in ROOT6.
Also remove rootcint from BuildFile.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #3597 from davidlt/fix-root6-ib

FWCore/PluginManager: gCINTMutex -> gInterpreterMutex