Make sl_list_length const correct

include/linklist-lib/singly-linked-list.h

@@ -89,7 +89,7 @@ void sl_list_free_full(SlList *list, void (*destroy_element)(void *));
  * @param list List
  * @return Length of list
  */
-uint32_t sl_list_length(SlList *list);
+uint32_t sl_list_length(const SlList *list);
 
 /**
  * @brief Get nth element in list.

src/singly-linked-list.c

@@ -117,7 +117,7 @@ void sl_list_free_full(SlList *list, void (*destroy_element)(void *))
 	}
 }
 
-uint32_t sl_list_length(SlList *list)
+uint32_t sl_list_length(const SlList *list)
 {
 	uint32_t count = 0;