alq_open_flags(): mark local td variable as unused

It is passed to the NDINIT() macro which ignores the thread argument
for some time.

Sponsored by:	The FreeBSD Foundation

sys/kern/kern_alq.c

@@ -431,7 +431,7 @@ int
 alq_open_flags(struct alq **alqp, const char *file, struct ucred *cred, int cmode,
     int size, int flags)
 {
-	struct thread *td;
+	struct thread *td __unused;
 	struct nameidata nd;
 	struct alq *alq;
 	int oflags;