gob: annotate debug.go so it's not normally built

So it's not included in the package by the go tool.

R=iant, fullung, rsc
CC=golang-dev
https://golang.org/cl/5563049

src/pkg/encoding/gob/debug.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Delete the next line to include this file in the gob package.
+// +build ignore
+
 package gob
 
 // This file is not normally included in the gob package.  Used only for debugging the package itself.