Add 2 missing includes

git-svn-id: http://root.cern.ch/svn/root/trunk@24305 27541ba8-7e3a-0410-8455-c3a389f83636

tutorials/tree/tree2a.C

+#include "TROOT.h"
 #include "TFile.h"
 #include "TTree.h"
 #include "TBrowser.h"
 #include "TH2.h"
+#include "TMath.h"
 #include "TRandom.h"
 #include "TCanvas.h"