kernel: priority_q: fix comparison of int of different signs

Clang 20.1.0 -Wsign-compare warning on int being compared with size_t
(unsigned int).

Signed-off-by: Alex Fabre <alex.fabre@rtone.fr>

kernel/include/priority_q.h

@@ -280,7 +280,7 @@ static ALWAYS_INLINE unsigned int z_priq_mq_best_queue_index(struct _priq_mq *pq
 
 static ALWAYS_INLINE void z_priq_mq_init(struct _priq_mq *q)
 {
-	for (int i = 0; i < ARRAY_SIZE(q->queues); i++) {
+	for (size_t i = 0; i < ARRAY_SIZE(q->queues); i++) {
 		sys_dlist_init(&q->queues[i]);
 	}