go/build: update syslist.go package comment

It's no longer generated by make.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5602047

src/pkg/go/build/syslist.go

-// Generated automatically by make.
+// Copyright 2011 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.
+
 package build
 
 const goosList = "darwin freebsd linux netbsd openbsd plan9 windows "