archive/tar: add dragonfly build tag

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/55720044

src/pkg/archive/tar/stat_atim.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux openbsd solaris
+// +build linux dragonfly openbsd solaris
 
 package tar
 

src/pkg/archive/tar/stat_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux darwin freebsd openbsd netbsd solaris
+// +build linux darwin dragonfly freebsd openbsd netbsd solaris
 
 package tar