drivers: staging: android: ion: ion_dummy_driver: include "linux/io.h"

Need add "linux/io.h" to pass compiling under metag architecture with
allmodconfig (which use the default 'virt_to_phys'), the related error:

    CC      drivers/staging/android/ion/ion_dummy_driver.o
  drivers/staging/android/ion/ion_dummy_driver.c: In function 'ion_dummy_init':
  drivers/staging/android/ion/ion_dummy_driver.c:81: error: implicit declaration of function 'virt_to_phys'

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>