COMP: Added virtual destructor to ctkDependency graph

That should fix the following warning:
warning: 'class ctkDependencyGraph' has virtual functions but non-virtual destructor

See http://www.cdash.org/CDash/viewBuildError.php?type=1&buildid=642781

Libs/Core/ctkDependencyGraph.h

@@ -36,7 +36,7 @@ class CTK_CORE_EXPORT ctkDependencyGraph
 {
 public:
   ctkDependencyGraph(int nvertices);
-  ~ctkDependencyGraph();
+  virtual ~ctkDependencyGraph();
   
   void printAdditionalInfo();
   void printGraph();