wayland/cursor-role: Add back priv variable

Rebase of previous patch that removed the priv variable shouldn't have,
since it was now used by other things. Add it back so it compiles again.

src/wayland/meta-wayland-surface-role-cursor.c

@@ -115,6 +115,10 @@ cursor_surface_role_assigned (MetaWaylandSurfaceRole *surface_role)
 {
   MetaWaylandSurface *surface =
     meta_wayland_surface_role_get_surface (surface_role);
+  MetaWaylandSurfaceRoleCursor *cursor_role =
+    META_WAYLAND_SURFACE_ROLE_CURSOR (surface_role);
+  MetaWaylandSurfaceRoleCursorPrivate *priv =
+    meta_wayland_surface_role_cursor_get_instance_private (cursor_role);
 
   wl_list_insert_list (&priv->frame_callbacks,
                        &surface->pending_frame_callback_list);