[libsigcplusplus] limit_refernce: =default copy/move operations.

[Murray Cumming <murrayc src gnome org>]

commit 259f20b3d7b43fa97f4a611aa727c65095d776c6
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 22 10:14:24 2016 +0200

    limit_refernce: =default copy/move operations.
    
    Because we really use at least the default copy constructor,
    so this makes that clearer.

 sigc++/limit_reference.h |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/limit_reference.h b/sigc++/limit_reference.h
index d038210..b57e4e0 100644
--- a/sigc++/limit_reference.h
+++ b/sigc++/limit_reference.h
@@ -53,11 +53,19 @@ class limit_reference
 public:
   using reference_type = typename std::remove_volatile_t<T_type>;
 
+  limit_reference() = delete;
+
   /** Constructor.
    * @param target The reference to limit.
    */
   explicit limit_reference(reference_type& target) : visited(target) {}
 
+  limit_reference(const limit_reference& src) = default;
+  limit_reference& operator=(const limit_reference& src) = default;
+
+  limit_reference(limit_reference&& src) = default;
+  limit_reference& operator=(limit_reference&& src) = default;
+
   /** Retrieve the entity to visit for visit_each().
    * Depending on the template specialization, this is either a derived reference, or
    * sigc::trackable& if T_type derives from sigc::trackable.