[libsigcplusplus] limit_reference: Make constructor explicit.

[Murray Cumming <murrayc src gnome org>]

commit 32f5839488242de4488d719bbe49a2294479baab
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 22 10:08:04 2016 +0200

    limit_reference: Make constructor explicit.

 sigc++/limit_reference.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/limit_reference.h b/sigc++/limit_reference.h
index 0c6b9a9..d038210 100644
--- a/sigc++/limit_reference.h
+++ b/sigc++/limit_reference.h
@@ -56,7 +56,7 @@ public:
   /** Constructor.
    * @param target The reference to limit.
    */
-  limit_reference(reference_type& target) : visited(target) {}
+  explicit limit_reference(reference_type& target) : visited(target) {}
 
   /** Retrieve the entity to visit for visit_each().
    * Depending on the template specialization, this is either a derived reference, or