unix: include export_test.go only during unix build

Fixes plan9 and windows build.

Change-Id: I3d913441a4f3f3f3da82ab74ef165b91d05eb8d3
Reviewed-on: https://go-review.googlesource.com/10078
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/export_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 dragonfly freebsd linux netbsd openbsd solaris
+
 package unix
 
 var Itoa = itoa