lib: add struct paddings to be able to maintain ABI compatibility

Aim for 8 pointers on each struct, wich exceptions:
- 12 pointers for already crowded/important ones
- 4 pointers for empty ones that are used for proxies; we are
  unlikely to add any virtual methods on these classes ever

lib/wp/proxy-interfaces.h

@@ -46,6 +46,9 @@ struct _WpPipewireObjectInterface
 
   gboolean (*set_param) (WpPipewireObject * self, const gchar * id,
       guint32 flags, WpSpaPod * param);
+
+  /*< private >*/
+  WP_PADDING(5)
 };
 
 WP_API