powerpc: Fix wrong defintion in boot/io.h

Fix wrong __IO_H definition in boot/io.h
Reported-by: Fernando Silveira <fsilveira@gmail.com>
Signed-off-by: Lucas Tanure <tanure@linux.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/boot/io.h

 #ifndef _IO_H
-#define __IO_H
+#define _IO_H
 
 #include "types.h"