vendor: Add missing file to github.com/syncthing/notify (#5042)

vendor/github.com/syncthing/notify/watcher_notimplemented.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2014-2018 The Notify Authors. All rights reserved.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+// +build !darwin,!linux,!freebsd,!dragonfly,!netbsd,!openbsd,!windows
+// +build !kqueue,!solaris
+
+package notify
+
+import "errors"
+
+// newWatcher stub.
+func newWatcher(chan<- EventInfo) watcher {
+	return watcherStub{errors.New("notify: not implemented")}
+}