core: fix const-correctness of queue functions

core/include/blue/core/queue.h

@@ -60,7 +60,7 @@ static inline b_queue_entry *b_queue_prev(const b_queue_entry *entry)
 	return entry->qe_prev;
 }
 
-BLUE_API size_t b_queue_length(b_queue *q);
+BLUE_API size_t b_queue_length(const b_queue *q);
 
 BLUE_API void b_queue_insert_before(
 	b_queue *q, b_queue_entry *entry, b_queue_entry *before);

core/queue.c

@@ -1,6 +1,6 @@
 #include <blue/core/queue.h>
 
-size_t b_queue_length(struct b_queue *q)
+size_t b_queue_length(const struct b_queue *q)
 {
 	size_t i = 0;
 	struct b_queue_entry *x = q->q_first;