go.crypto/curve25519: fix incorrect build tag

The build tag was incorrect. CL 6458091 exposed the mistake.

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/6460067

curve25519/curve25519.go

@@ -2,12 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package curve25519
-
 // We have a implementation in amd64 assembly so this code is only run on
 // non-amd64 platforms.
 // +build !amd64
 
+package curve25519
+
 import (
 	"math/big"
 )