[i386] Add missing #include <ipxe/io.h> in pic8259.h

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/include/pic8259.h

 #ifndef PIC8259_H
 #define PIC8259_H
 
+#include <ipxe/io.h>
+
 /* For segoff_t */
 #include "realmode.h"