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/gtkstylecontext.h

@@ -52,6 +52,12 @@ struct _GtkStyleContextClass
   GObjectClass parent_class;
 
   void (* changed) (GtkStyleContext *context);
+
+  /* Padding for future expansion */
+  void (*_gtk_reserved1) (void);
+  void (*_gtk_reserved2) (void);
+  void (*_gtk_reserved3) (void);
+  void (*_gtk_reserved4) (void);
 };
 
 /* Default set of properties that GtkStyleContext may contain */