reference to undefined struct wl_object in wayland-util.h

In wayland-util.h:

union wl_argument {
	int32_t i;           /**< `int`    */
	uint32_t u;          /**< `uint`   */
	wl_fixed_t f;        /**< `fixed`  */
	const char *s;       /**< `string` */
	struct wl_object *o; /**< `object` */
	uint32_t n;          /**< `new_id` */
	struct wl_array *a;  /**< `array`  */
	int32_t h;           /**< `fd`     */
};

wl_object is not defined. There is a wl_object in server code, but I am guessing that is not the same.

It looks like perhaps the client wl_object type was supposed to become an opaque struct in d94a8722, where other references to it were replaced with void pointers instead.

I'm generating Cython headers from wayland's C headers, and Cython is not happy about the undefined wl_object. I'm less familiar with C so I don't know if it is an issue for C code ordinarily, but it seems like probably an oversight in any case.