nfsd: Fix old-style function definition

Fix warning:

fs/nfsd/nfssvc.c:604:6: warning: old-style function definition [-Wold-style-definition]
 bool i_am_nfsd()
      ^~~~~~~~~
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NMa Feng <mafeng.ma@huawei.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfssvc.c

@@ -601,7 +601,7 @@ static const struct svc_serv_ops nfsd_thread_sv_ops = {
 	.svo_module		= THIS_MODULE,
 };
 
-bool i_am_nfsd()
+bool i_am_nfsd(void)
 {
 	return kthread_func(current) == nfsd;
 }