[glibmm] sigc3: Binding: TransformProp: Don't use sigc::functor_base.

[Murray Cumming <murrayc src gnome org>]

commit 53ceee1d8f4967d4fc3a9ece73a8a273583aae5e
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Nov 10 13:02:11 2016 +0100

    sigc3: Binding: TransformProp: Don't use sigc::functor_base.

 glib/src/binding.hg |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/glib/src/binding.hg b/glib/src/binding.hg
index cb53709..730baad 100644
--- a/glib/src/binding.hg
+++ b/glib/src/binding.hg
@@ -391,10 +391,9 @@ private:
   // The functor TransformProp can be implicitly converted to a SlotTransform
   // and used in a call to bind_property_value().
   template <typename T_from, typename T_to>
-  class TransformProp : public sigc::functor_base
+  class TransformProp
   {
   public:
-    using result_type = bool;
     using SlotTypedTransform = sigc::slot<bool(const T_from&, T_to&)>;
 
     TransformProp(const SlotTypedTransform& slot) : typed_transform(slot) {}