cmd/dist: refresh deps.go after recent package io changes

Change-Id: Iaa960c85011289e047c64b53cf610838eb50332d
Reviewed-on: https://go-review.googlesource.com/70073
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/deps.go

@@ -681,6 +681,7 @@ var builddeps = map[string][]string{
 	"io": {
 		"errors",      // io
 		"sync",        // io
+		"sync/atomic", // io
 	},
 
 	"io/ioutil": {