Resolve compiler warning about _tree.

Add unused routines in order to suppress the error message:

include/RooTreeData.h:33:10: error: private field '_tree' is not used [-Werror,-Wunused-private-field]
  TTree *_tree ;           // TTree holding the data points
         ^

roofit/roofitcore/inc/RooTreeData.h

@@ -26,7 +26,8 @@ class RooTreeData : public RooAbsData {
 public:
 
   // Constructors, factory methods etc.
-  RooTreeData() {} ; 
+  RooTreeData() : _tree(0) {} ; 
+  const TTree *GetTree() const { return _tree; }
   
 private: