Fix unused 'j' build failure

Variable 'j' is unused in this test and clang emits a warning. `-Werror`
is enabled, so this warning is actually emitted as an error.

Author: etcwilde

tests/dispatch_apply.c

@@ -65,6 +65,7 @@ static void busythread(void *ignored)
 		j += i;
 		i += 1;
 	}
+	(void)j;
 
 	OSAtomicIncrement32(&busy_threads_finished);
 }