Fix build on Plan 9

This fixes build for programs that import fsnotify but fsnotify is not
absolutely necessary for the program to be useful.

fen.go

@@ -31,7 +31,7 @@ func (w *Watcher) Add(name string) error {
 	return nil
 }
 
-// Remove stops watching the the named file or directory (non-recursively).
+// Remove stops watching the named file or directory (non-recursively).
 func (w *Watcher) Remove(name string) error {
 	return nil
 }

fsnotify.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !plan9
-
 // Package fsnotify provides a platform-independent interface for file system notifications.
 package fsnotify
 

kqueue.go

@@ -99,7 +99,7 @@ func (w *Watcher) Add(name string) error {
 	return err
 }
 
-// Remove stops watching the the named file or directory (non-recursively).
+// Remove stops watching the named file or directory (non-recursively).
 func (w *Watcher) Remove(name string) error {
 	name = filepath.Clean(name)
 	w.mu.Lock()

plan9.go

@@ -0,0 +1,35 @@
+// Copyright 2020 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 plan9
+
+package fsnotify
+
+import "syscall"
+
+// Watcher watches a set of files, delivering events to a channel.
+type Watcher struct {
+	Events chan Event
+	Errors chan error
+}
+
+// NewWatcher establishes a new watcher with the underlying OS and begins waiting for events.
+func NewWatcher() (*Watcher, error) {
+	return nil, syscall.EPLAN9
+}
+
+// Close removes all watches and closes the events channel.
+func (w *Watcher) Close() error {
+	return syscall.EPLAN9
+}
+
+// Add starts watching the named file or directory (non-recursively).
+func (w *Watcher) Add(name string) error {
+	return syscall.EPLAN9
+}
+
+// Remove stops watching the named file or directory (non-recursively).
+func (w *Watcher) Remove(name string) error {
+	return syscall.EPLAN9
+}

windows.go

@@ -81,7 +81,7 @@ func (w *Watcher) Add(name string) error {
 	return <-in.reply
 }
 
-// Remove stops watching the the named file or directory (non-recursively).
+// Remove stops watching the named file or directory (non-recursively).
 func (w *Watcher) Remove(name string) error {
 	in := &input{
 		op:    opRemoveWatch,