eliminated some GCC 4.x warnings

modules/ml/src/precomp.hpp

@@ -356,6 +356,7 @@ namespace cv
         DTreeBestSplitFinder(){ tree = 0; node = 0; }
         DTreeBestSplitFinder( CvDTree* _tree, CvDTreeNode* _node);
         DTreeBestSplitFinder( const DTreeBestSplitFinder& finder, Split );
+        virtual ~DTreeBestSplitFinder() {}
         virtual void operator()(const BlockedRange& range);
         void join( DTreeBestSplitFinder& rhs );
         Ptr<CvDTreeSplit> bestSplit;