Mark conntrack/fake as linux-only, add non-OS doc.go

pkg/proxy/conntrack/doc.go

@@ -0,0 +1,17 @@
+/*
+Copyright 2024 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 conntrack

pkg/proxy/conntrack/fake.go

@@ -1,3 +1,6 @@
+//go:build linux
+// +build linux
+
 /*
 Copyright 2016 The Kubernetes Authors.