expo: Make scene_obj_find() take a const scene

This does not change the scene, so mark the pointer const.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/scene.c b/boot/scene.c
index e523333..b4c36c4 100644
--- a/boot/scene.c
+++ b/boot/scene.c
@@ -79,7 +79,7 @@
 	return count;
 }
 
-void *scene_obj_find(struct scene *scn, uint id, enum scene_obj_t type)
+void *scene_obj_find(const struct scene *scn, uint id, enum scene_obj_t type)
 {
 	struct scene_obj *obj;