[libsigcplusplus] slot_list: Make list_ private, not protected.

[Murray Cumming <murrayc src gnome org>]

commit 4ac87657aa3fccf9d825d9e3eabdc0fabaaf2eeb
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Mar 30 16:13:03 2016 +0200

    slot_list: Make list_ private, not protected.

 sigc++/signal.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/signal.h b/sigc++/signal.h
index cd87b35..47f81ad 100644
--- a/sigc++/signal.h
+++ b/sigc++/signal.h
@@ -237,7 +237,7 @@ struct slot_list
     erase(--tmp_);
   }
 
-protected:
+private:
   internal::signal_impl* list_;
 };