[libsigcplusplus] TupleVisitorVisitEach and TupleVisitorVisitEach: Use constexpr.

[Murray Cumming <murrayc src gnome org>]

commit a231515b9981035a03e71725647e4c5f2b11e587
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Mar 8 14:16:58 2016 +0100

    TupleVisitorVisitEach and TupleVisitorVisitEach: Use constexpr.
    
    This would seem to be necessary if the tuple utils are ever fully
    constexpr.

 sigc++/adaptors/bind.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index 4ad2c37..65281f7 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -87,6 +87,7 @@ struct TransformEachInvoker
 {
   //We take T_element as non-const because invoke() is not const.
   //TODO: Take element as T_element&& ?
+  constexpr
   static
   decltype(auto)
   transform(T_element& element) {
@@ -223,6 +224,7 @@ template<typename T_element>
 struct TupleVisitorVisitEach
 {
   template<typename T_action>
+  constexpr
   static
   void
   visit(const T_element& element, const T_action& action)