[gtk+/center-box] Keep GtkCenterBox struct private

[Matthias Clasen <matthiasc src gnome org>]

commit 1c17f382a595b0e25aa95024a3bd21d7e5af8ce4
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Jun 4 12:45:21 2017 -0400

    Keep GtkCenterBox struct private
    
    No need to derive from this.

 gtk/gtkcenterbox.c |   15 +++++++++++++++
 gtk/gtkcenterbox.h |   14 --------------
 2 files changed, 15 insertions(+), 14 deletions(-)
---
diff --git a/gtk/gtkcenterbox.c b/gtk/gtkcenterbox.c
index 9f34898..c65642c 100644
--- a/gtk/gtkcenterbox.c
+++ b/gtk/gtkcenterbox.c
@@ -38,6 +38,21 @@
 #include "gtkbuildable.h"
 #include "gtksizerequest.h"
 
+struct _GtkCenterBox
+{
+  GtkWidget parent_instance;
+
+  GtkWidget *start_widget;
+  GtkWidget *center_widget;
+  GtkWidget *end_widget;
+};
+
+
+struct _GtkCenterBoxClass
+{
+  GtkWidgetClass parent_class;
+};
+
 
 static void gtk_center_box_buildable_init (GtkBuildableIface *iface);
 
diff --git a/gtk/gtkcenterbox.h b/gtk/gtkcenterbox.h
index bf13f39..709ac60 100644
--- a/gtk/gtkcenterbox.h
+++ b/gtk/gtkcenterbox.h
@@ -34,20 +34,6 @@
 typedef struct _GtkCenterBox             GtkCenterBox;
 typedef struct _GtkCenterBoxClass        GtkCenterBoxClass;
 
-struct _GtkCenterBox
-{
-  GtkWidget parent_instance;
-
-  GtkWidget *start_widget;
-  GtkWidget *center_widget;
-  GtkWidget *end_widget;
-};
-
-struct _GtkCenterBoxClass
-{
-  GtkWidgetClass parent_class;
-};
-
 GDK_AVAILABLE_IN_3_92
 GType      gtk_center_box_get_type (void) G_GNUC_CONST;