log/syslog: add stub for Plan 9

Proper local system log semantics still need to be
created for Plan 9. In the meantime, the test suite
(viz., exp/gotype) expects there to be some Go
source for each import path. Thus, here is a stub,
equivalent to syslog_windows, for this purpose.

R=golang-dev, rsc, alex.brainman
CC=golang-dev
https://golang.org/cl/7000062

src/pkg/log/syslog/syslog_plan9.go

+// Copyright 2012 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 syslog provides a simple interface to the system log service.
+package syslog
+
+// BUG(akumar): This package is not implemented on Plan 9 yet.