#include "go.h" (or "gg.h")

becomes

#include <u.h>
#include <libc.h>
#include "go.h"

so that go.y can #include <stdio.h>
after <u.h> but before "go.h".
This is necessary on Plan 9.

R=ken2
CC=golang-dev
https://golang.org/cl/4971041