x86: i8259: Add missing include file

i8259.h uses inb/outb and thus needs to include asm/io.h to avoid the
following build error, as seen with x86_64:defconfig and CONFIG_SMP=n.

  In file included from drivers/rtc/rtc-cmos.c:45:0:
  arch/x86/include/asm/i8259.h: In function 'inb_pic':
  arch/x86/include/asm/i8259.h:32:24: error:
	implicit declaration of function 'inb'

  arch/x86/include/asm/i8259.h: In function 'outb_pic':
  arch/x86/include/asm/i8259.h:45:2: error:
	implicit declaration of function 'outb'
Reported-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Suggested-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Fixes: 447ae316 ("x86: Don't include linux/irq.h from asm/hardirq.h")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/i8259.h

@@ -3,6 +3,7 @@
 #define _ASM_X86_I8259_H
 
 #include <linux/delay.h>
+#include <asm/io.h>
 
 extern unsigned int cached_irq_mask;