[libsigcplusplus] signal_impl: Doesn't need to derive from notifiable.

[Murray Cumming <murrayc src gnome org>]

commit 4acf00f32f578cc7ee196edb9a6f75f684a0ffb9
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Apr 28 12:25:50 2016 +0200

    signal_impl: Doesn't need to derive from notifiable.

 sigc++/signal_base.h |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/sigc++/signal_base.h b/sigc++/signal_base.h
index de72647..0c491fc 100644
--- a/sigc++/signal_base.h
+++ b/sigc++/signal_base.h
@@ -44,8 +44,7 @@ namespace internal
  * invalid slots from the list.
  */
 struct SIGC_API signal_impl
- : public notifiable,
-   public std::enable_shared_from_this<signal_impl>
+ : public std::enable_shared_from_this<signal_impl>
 {
   using size_type = std::size_t;
   using slot_list = std::list<slot_base>;