sh: Fix up headers_check regression.

linux/mmzone.h isn't exported, kill it off from asm/setup.h and simply
deal with it in the places that have a dependency instead.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/include/asm/setup.h

 #ifndef _SH_SETUP_H
 #define _SH_SETUP_H
 
-#include <linux/mmzone.h>
-
 #define COMMAND_LINE_SIZE 256
 
 #ifdef __KERNEL__
-
 /*
  * This is set up by the setup-routine at boot-time
  */