gc: shuffle #includes

#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
diff --git a/src/cmd/gc/typecheck.c b/src/cmd/gc/typecheck.c
index ef900d0..7d67e1e 100644
--- a/src/cmd/gc/typecheck.c
+++ b/src/cmd/gc/typecheck.c
@@ -10,6 +10,8 @@
  * rewrites n->op to be more specific in some cases.
  */
 
+#include <u.h>
+#include <libc.h>
 #include "go.h"
 
 static void	implicitstar(Node**);