[libsigcplusplus] TrackObjVisitEach: Use constexpr, as in TupleVisitorVisitEach.

[Murray Cumming <murrayc src gnome org>]

commit 0721f45ddcd4cf1fbbf076b07555c8e1d30f3aa6
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Apr 13 12:56:55 2016 +0200

    TrackObjVisitEach: Use constexpr, as in TupleVisitorVisitEach.

 sigc++/adaptors/track_obj.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/adaptors/track_obj.h b/sigc++/adaptors/track_obj.h
index 5fad4cf..d3edd9e 100644
--- a/sigc++/adaptors/track_obj.h
+++ b/sigc++/adaptors/track_obj.h
@@ -118,7 +118,7 @@ private:
   struct TrackObjVisitForEach
   {
     template <typename T_action>
-    static void visit(const T_element& element, T_action&& action)
+    constexpr static void visit(const T_element& element, T_action&& action)
     {
       sigc::visit_each(std::forward<T_action>(action), element);
     }