sig_ast_checksusp(): mark the local p as __diagused

It is only used to assert that the (current) process is locked

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/kern_sig.c

@@ -3262,7 +3262,7 @@ postsig(int sig)
 int
 sig_ast_checksusp(struct thread *td)
 {
-	struct proc *p;
+	struct proc *p __diagused;
 	int ret;
 
 	p = td->td_proc;