Silence warnings about unused variables when RACCT is defined but RCTL

is not.

Reported by:	Dries Michiels <driesm.michiels@gmail.com>
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

sys/kern/kern_jail.c

@@ -3988,8 +3988,10 @@ prison_racct_attach(struct prison *pr)
 static void
 prison_racct_modify(struct prison *pr)
 {
+#ifdef RCTL
 	struct proc *p;
 	struct ucred *cred;
+#endif
 	struct prison_racct *oldprr;
 
 	ASSERT_RACCT_ENABLED();