[libsigcplusplus] trackable.cc: Use auto instead of specifying iterator type.

[Murray Cumming <murrayc src gnome org>]

commit ae3d3cbbaf0fa6dc31143364d6656cc6fc4183de
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 18:11:53 2016 +0200

    trackable.cc: Use auto instead of specifying iterator type.

 sigc++/trackable.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/trackable.cc b/sigc++/trackable.cc
index d2a8990..8cbef82 100644
--- a/sigc++/trackable.cc
+++ b/sigc++/trackable.cc
@@ -142,7 +142,7 @@ trackable_callback_list::clear()
 void
 trackable_callback_list::remove_callback(notifiable* data)
 {
-  for (callback_list::iterator i = callbacks_.begin(); i != callbacks_.end(); ++i)
+  for (auto i = callbacks_.begin(); i != callbacks_.end(); ++i)
   {
     auto& callback = *i;
     if (callback.data_ == data && callback.func_ != nullptr)