wayland-util.h: add forward declaration for wl_object

The definition of wl_argument in wayland-util.h references wl_object,
so wl_object ought to be defined in wayland-util.h. This resolves
gitlab issue #78.

Fixes: https://gitlab.freedesktop.org/wayland/wayland/issues/78

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

src/wayland-util.h

@@ -68,6 +68,20 @@ extern "C" {
 #define WL_PRINTF(x, y)
 #endif
 
+/** \class wl_object
+ *
+ * \brief A protocol object.
+ *
+ * A `wl_object` is an opaque struct identifying the protocol object
+ * underlying a `wl_proxy` or `wl_resource`.
+ *
+ * \note Functions accessing a `wl_object` are not normally used by client code.
+ * Clients should normally use the higher level interface generated by the
+ * scanner to interact with compositor objects.
+ *
+ */
+struct wl_object;
+
 /**
  * Protocol message signature
  *