[libsigcplusplus] Some perfect-forwarding.

[Murray Cumming <murrayc src gnome org>]

commit 972936cb3ae2d3598babe9727e6893dabcf51e68
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 10:34:46 2016 +0200

    Some perfect-forwarding.

 sigc++/adaptors/bind.h      |    4 ++--
 sigc++/adaptors/track_obj.h |    4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index 2365526..e055a4f 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -216,9 +216,9 @@ template <typename T_element>
 struct TupleVisitorVisitEach
 {
   template <typename T_action>
-  constexpr static void visit(const T_element& element, const T_action& action)
+  constexpr static void visit(const T_element& element, T_action&& action)
   {
-    sigc::visit_each(action, element);
+    sigc::visit_each(std::forward<T_action>(action), element);
   }
 };
 
diff --git a/sigc++/adaptors/track_obj.h b/sigc++/adaptors/track_obj.h
index d379e45..5fad4cf 100644
--- a/sigc++/adaptors/track_obj.h
+++ b/sigc++/adaptors/track_obj.h
@@ -118,9 +118,9 @@ private:
   struct TrackObjVisitForEach
   {
     template <typename T_action>
-    static void visit(const T_element& element, const T_action& action)
+    static void visit(const T_element& element, T_action&& action)
     {
-      sigc::visit_each(action, element);
+      sigc::visit_each(std::forward<T_action>(action), element);
     }
   };
 };