cmd/compile: add copyright notice to util.go

util.go was originally added in golang.org/cl/4851, and later moved to
its current location in golang.org/cl/10287.

Change-Id: I10b4941d42ae1ff2e78990c497c1347bbbae4e3d
Reviewed-on: https://go-review.googlesource.com/19851Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/compile/internal/gc/util.go

+// Copyright 2015 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 gc
 
 import (