skip MetaWaylandFrameCallback for gtk-doc

gtk-doc has a bug in its parser where it doesn't allow the first field
of a struct to have something of the form `struct foo bar;`.

See also https://github.com/GNOME/gtk-doc/blob/master/gtkdoc/common.py#L236

src/wayland/meta-wayland-private.h

@@ -35,6 +35,9 @@
 
 typedef struct _MetaXWaylandSelection MetaXWaylandSelection;
 
+/**
+ * MetaWaylandFrameCallback: (skip)
+ */
 typedef struct
 {
   struct wl_list link;