Fix undefined symbols for ccl::Node with some compilers and build options

Differential Revision: https://developer.blender.org/D5590

intern/cycles/graph/node.h

@@ -33,7 +33,7 @@ struct Transform;
 
 struct Node {
   explicit Node(const NodeType *type, ustring name = ustring());
-  virtual ~Node();
+  virtual ~Node() = 0;
 
   /* set values */
   void set(const SocketType &input, bool value);