add  #pragma link C++ global ROOT_TMath as suggested by Philippe. This avoids via the header guard to re-include the definition of TMath.h when running in CINT. 
This fixes a crash observed running stressFit.cxx in interpreted mode


git-svn-id: http://root.cern.ch/svn/root/trunk@23686 27541ba8-7e3a-0410-8455-c3a389f83636