fscache: fix build on !CONFIG_SYSCTL

Commit 8b8edefa (fscache: convert object to use workqueue instead of
slow-work) made fscache_exit() call unregister_sysctl_table()
unconditionally breaking build when sysctl is disabled.  Fix it by
putting it inside CONFIG_SYSCTL.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: David Howells <dhowells@redhat.com>

fs/fscache/main.c

@@ -186,7 +186,9 @@ static void __exit fscache_exit(void)
 
 	kobject_put(fscache_root);
 	kmem_cache_destroy(fscache_cookie_jar);
+#ifdef CONFIG_SYSCTL
 	unregister_sysctl_table(fscache_sysctl_header);
+#endif
 	fscache_proc_cleanup();
 	destroy_workqueue(fscache_op_wq);
 	destroy_workqueue(fscache_object_wq);