cmd/godoc: s/ignore/appengine/ in appinit.go +build constraint

R=golang-dev
CC=golang-dev
https://golang.org/cl/5784072

src/cmd/godoc/appinit.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
+// +build appengine
 
 package main