[gtkmm] Button: Remove unnnecessary BuiltinStockID declaration

[Murray Cumming <murrayc src gnome org>]

commit 15efeb9a8518323ab706754ad33f640b81e60053
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Sep 26 13:31:00 2013 +0200

    Button: Remove unnnecessary BuiltinStockID declaration

 gtk/src/button.hg |    2 --
 1 files changed, 0 insertions(+), 2 deletions(-)
---
diff --git a/gtk/src/button.hg b/gtk/src/button.hg
index c709b57..446f0a9 100644
--- a/gtk/src/button.hg
+++ b/gtk/src/button.hg
@@ -30,8 +30,6 @@ _PINCLUDE(gtkmm/private/bin_p.h)
 namespace Gtk
 {
 
-namespace Stock { struct BuiltinStockID; }
-
 //TODO: Derive from (and implement) Actionable when we can break ABI.
 
 /** A widget that creates a signal when clicked on.