kernel: Make statements evaluate boolean expressions

MISRA-C requires that the if statement has essentially Boolean type.

MISRA-C rule 14.4

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

kernel/mempool.c

@@ -124,7 +124,7 @@ void k_mem_pool_free_id(struct k_mem_block_id *id)
 
 	need_sched = z_unpend_all(&p->wait_q);
 
-	if (need_sched) {
+	if (need_sched != 0) {
 		z_reschedule(&lock, key);
 	} else {
 		k_spin_unlock(&lock, key);