Revert "Remove forward boosts declarations"

This reverts commit 50d2a986.
The vtkInfovisBoostGraphAlgorithms module uses these forward
declarations and friendship.

Common/DataModel/vtkGraph.h

@@ -224,6 +224,15 @@ class vtkVariant;
 class vtkVariantArray;
 
 //BTX
+// Forward declare some boost stuff even if boost wrappers
+// are turned off.
+namespace boost
+{
+  class vtk_edge_iterator;
+  class vtk_out_edge_pointer_iterator;
+  class vtk_in_edge_pointer_iterator;
+}
+
 // Edge structures.
 struct vtkEdgeBase
 {
@@ -676,6 +685,9 @@ protected:
   friend class vtkEdgeListIterator;
   friend class vtkInEdgeIterator;
   friend class vtkOutEdgeIterator;
+  friend class boost::vtk_edge_iterator;
+  friend class boost::vtk_in_edge_pointer_iterator;
+  friend class boost::vtk_out_edge_pointer_iterator;
   //ETX
 
   // Description: