util/iptables: fix cross-build failures due to syscall.Flock()

Fixes: https://github.com/kubernetes/kubernetes/issues/45554

pkg/util/iptables/iptables_unsupported.go

@@ -0,0 +1,32 @@
+// +build !linux
+
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package iptables
+
+import (
+	"fmt"
+	"os"
+)
+
+func grabIptablesLocks(lockfilePath string) (iptablesLocker, error) {
+	return nil, fmt.Errorf("iptables unsupported on this platform")
+}
+
+func grabIptablesFileLock(f *os.File) error {
+	return fmt.Errorf("iptables unsupported on this platform")
+}