[libsigc++2/variadic_bind4] bind_functor: Make bound_ private.

[Murray Cumming <murrayc src gnome org>]

commit 3bbeffd4054f54c3612349f61d0aee678ed1e180
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Mar 3 22:50:42 2016 +0100

    bind_functor: Make bound_ private.

 sigc++/adaptors/bind.h |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index 390d42b..238cb4d 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -159,11 +159,10 @@ struct bind_functor : public adapts<T_functor>
     : adapts<T_functor>(_A_func), bound_(_A_bound...)
     {}
 
-  //TODO: Should this be private?
+private:
   /// The arguments bound to the functor.
   std::tuple<bound_argument<T_bound>...> bound_;
 
-private:
   template<class T_specific, class T, std::size_t... Is>
   decltype(auto)
   call_functor_operator_parentheses(T&& tuple,