[libsigcplusplus] self_and_iter: Take the iterator by const ref and store it as const.

[Murray Cumming <murrayc src gnome org>]

commit 48611e840e9a00937026f3d478889380b0efc158
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 15 16:53:15 2016 +0200

    self_and_iter: Take the iterator by const ref and store it as const.
    
    This is less generically useful, but this isn't a generally used class.

 sigc++/signal_base.cc |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/sigc++/signal_base.cc b/sigc++/signal_base.cc
index 8d7c357..4916002 100644
--- a/sigc++/signal_base.cc
+++ b/sigc++/signal_base.cc
@@ -29,9 +29,9 @@ namespace internal
 struct self_and_iter : public notifiable
 {
   signal_impl* self_;
-  signal_impl::iterator_type iter_;
+  const signal_impl::iterator_type iter_;
 
-  self_and_iter(signal_impl* self, signal_impl::iterator_type iter) : self_(self), iter_(iter) {}
+  self_and_iter(signal_impl* self, const signal_impl::iterator_type& iter) : self_(self), iter_(iter) {}
 };
 
 signal_impl::signal_impl() : ref_count_(0), exec_count_(0), deferred_(false)