Include fix to make TFormula thread safe.

Merged in cms-sw/root#24.
Merge pull request #1055 from degano/root_threadFix_72X

Include fix to make TFormula thread safe.
Merge remote branch 'origin/IB/CMSSW_7_2_X/stable' into IB/CMSSW_7_0_X/stable
Merge remote branch 'origin/IB/CMSSW_7_3_X/stable' into IB/CMSSW_7_0_X/stable