Update lv_list.c

lv_objx/lv_list.c

@@ -472,7 +472,7 @@ lv_obj_t * lv_list_get_next_btn(const lv_obj_t * list, lv_obj_t * prev_btn)
  * @param btn pointer to a list element (button)
  * @return the index of the button in the list, or -1 of the button not in this list
  */
-int lv_list_get_btn_index(const lv_obj_t * list, const lv_obj_t * btn)
+int32_t lv_list_get_btn_index(const lv_obj_t * list, const lv_obj_t * btn)
 {
 	int index = 0;
 	lv_obj_t * e = lv_list_get_next_btn(list, NULL);