Use ROOT’s Cint Mutex to protect access to CINT structures

When requested to load a dictionary, we have to manipulate some Cint data structures to say what file it comes from. When used in a threaded environment, ROOT requires a lock be taken before accessing these structures.
Merge pull request #2230 from Dr15Jones/useCintLockBeforeUpdating

Use ROOT’s Cint Mutex to protect access to CINT structures