move establishVPN to a platform specific file

client/go/outline/vpn_others.go

@@ -0,0 +1,22 @@
+// Copyright 2024 The Outline 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.
+
+//go:build !linux
+
+package outline
+
+import "errors"
+
+func establishVPN(configStr string) error { return errors.ErrUnsupported }
+func closeVPN() error                     { return errors.ErrUnsupported }