[DF] Include explicitly TROOT.h in RDataFrame.hxx
[DF] Include explicitly TROOT.h in RDataFrame.hxx

in order to allow to call ROOT::EnableImplicitMT() without including
that header.

olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,olifre/root