Add missing isc_rwlock_destroy() for keylist_lock in dnssec-signzone

The keylist_lock rwlock is initialized at startup but never destroyed
on exit, unlike the sibling namelock mutex which is properly cleaned up.

(cherry picked from commit 5dc19a7d92)

bin/dnssec/dnssec-signzone.c

@@ -4182,6 +4182,7 @@ main(int argc, char *argv[]) {
 			    &sign_finish);
 	}
 	isc_mutex_destroy(&namelock);
+	isc_rwlock_destroy(&keylist_lock);
 
 	rcu_barrier();