[libsigc++2/variadic_bind4] adaptor_functor: Do some perfect forwarding.

[Murray Cumming <murrayc src gnome org>]

commit e99372da50a06aaf1cf56173244c9266ac6f5ce2
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Mar 3 10:00:26 2016 +0100

    adaptor_functor: Do some perfect forwarding.

 sigc++/adaptors/adaptor_trait.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/sigc++/adaptors/adaptor_trait.h b/sigc++/adaptors/adaptor_trait.h
index 9c20d0c..aadf7e8 100644
--- a/sigc++/adaptors/adaptor_trait.h
+++ b/sigc++/adaptors/adaptor_trait.h
@@ -92,8 +92,8 @@ struct adaptor_functor : public adaptor_base
    */
   template <class... T_arg>
   decltype(auto)
-  operator()(T_arg... _A_arg) const
-    { return functor_(_A_arg...); }
+  operator()(T_arg&&... _A_arg) const
+    { return functor_(std::forward<T_arg>(_A_arg)...); }
 
   /// Constructs an invalid functor.
   adaptor_functor()