os/signal: do not build on Plan 9

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5503057

src/pkg/os/signal/signal.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux netbsd openbsd windows
+
 // Package signal implements operating system-independent signal handling.
 package signal
 

src/pkg/os/signal/signal_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux netbsd openbsd windows
+
 package signal
 
 import (