test: add missing copyright header

Change-Id: Ia64535492515f725fe3c4b59ea300363a0c4ce10
Reviewed-on: https://go-review.googlesource.com/107136Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/codegen/condmove.go

 // asmcheck
 
+// Copyright 2018 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.
+
 package codegen
 
 func cmovint(c int) int {