vendor: add kcp-go package

vendor/golang.org/x/net/ipv4/bpfopt_stub.go

@@ -0,0 +1,16 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !linux
+
+package ipv4
+
+import "golang.org/x/net/bpf"
+
+// SetBPF attaches a BPF program to the connection.
+//
+// Only supported on Linux.
+func (c *dgramOpt) SetBPF(filter []bpf.RawInstruction) error {
+	return errOpNoSupport
+}