go.crypto/ssh/terminal: enable freebsd build

syscall.Termios, which was the only thing breaking the build, is
available in go tip now
(https://code.google.com/p/go/source/detail?r=873d664b00ec)

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/51690043

util.go

diff --git a/util.go b/util.go
index 8df94f5..84c7d1e 100644
--- a/util.go
+++ b/util.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,!appengine darwin
+// +build linux,!appengine darwin freebsd
 
 // Package terminal provides support functions for dealing with terminals, as
 // commonly found on UNIX systems.

util_bsd.go

diff --git a/util_bsd.go b/util_bsd.go
index 1654453..91b5834 100644
--- a/util_bsd.go
+++ b/util_bsd.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 darwin
+// +build darwin freebsd
 
 package terminal