expo: Export scene_menuitem_find() for use in internal code

Make this function available to other expo code so we can use it to look
up a menu item.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/scene_menu.c b/boot/scene_menu.c
index 57ffb52..602fe24 100644
--- a/boot/scene_menu.c
+++ b/boot/scene_menu.c
@@ -33,8 +33,8 @@
 		scene_menuitem_destroy(item);
 }
 
-static struct scene_menitem *scene_menuitem_find(struct scene_obj_menu *menu,
-						 int id)
+struct scene_menitem *scene_menuitem_find(const struct scene_obj_menu *menu,
+					  int id)
 {
 	struct scene_menitem *item;