add missing header

git-svn-id: http://root.cern.ch/svn/root/trunk@18402 27541ba8-7e3a-0410-8455-c3a389f83636

test/dt_DrawTest.C

 #include "TTree.h"
 #include "TFile.h"
 #include "TList.h"
+#include "TROOT.h"
 #include <TBenchmark.h>
 
 int gHasLibrary = kFALSE;