Fix sparse warning:

fs/nfsd/filecache.c:55:25: warning:
	symbol 'nfsd_filecache_wq' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>