shell: fix set_transient

In the wl_shell_surface migration, I forgot to correct one cast in
shell_surface_set_transient(). 'parent_resource' is not a (struct
wlsc_surface *) but (struct shell_surface *).

This bug corrupts a wlsc_surface::output field, which later (in my
experiments) leads to a segmentation fault in surface_frame().

Fix the casts.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

compositor/shell.c

@@ -312,7 +312,8 @@ shell_surface_set_transient(struct wl_client *client,
 {
 	struct shell_surface *shsurf = resource->data;
 	struct wlsc_surface *es = shsurf->surface;
-	struct wlsc_surface *pes = parent_resource->data;
+	struct shell_surface *pshsurf = parent_resource->data;
+	struct wlsc_surface *pes = pshsurf->surface;
 
 	/* assign to parents output  */
 	es->output = pes->output;