plan9: skip pwd_plan9_go15.go unless on plan9

Fixes build.

Change-Id: Ib090c951b192ec5d4e145aa2d302d140f99b1417
Reviewed-on: https://go-review.googlesource.com/10099
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

plan9/pwd_go15_plan9.go

@@ -0,0 +1,21 @@
+// Copyright 2015 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 go1.5
+
+package plan9
+
+import "syscall"
+
+func fixwd() {
+	syscall.Fixwd()
+}
+
+func Getwd() (wd string, err error) {
+	return syscall.Getwd()
+}
+
+func Chdir(path string) error {
+	return syscall.Chdir(path)
+}