gtk: add missing class padding

Sorry for this late ABI break, but the newly added style classes
definitely need padding, so I can just as well add missing padding
globally.

gtk/gtkaspectframe.h

@@ -59,6 +59,12 @@ struct _GtkAspectFrame
 struct _GtkAspectFrameClass
 {
   GtkFrameClass parent_class;
+
+  /* Padding for future expansion */
+  void (*_gtk_reserved1) (void);
+  void (*_gtk_reserved2) (void);
+  void (*_gtk_reserved3) (void);
+  void (*_gtk_reserved4) (void);
 };