s390: vfio-ap: include <asm/facility> for test_facility()

The driver uses test_facility(), but does not include the
corresponding include file explicitly. The driver currently builds
only thanks to the following include chain:

  vfio_ap_drv.c
    <linux/module.h>
      <linux/elf.h>
        <asm/elf.h>
	  <linux/compat.h>
	    <asm/uaccess.h>
	      <asm/facility.h>

Files should not rely on such fragile implicit includes.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>