xen: xlate_mmu: add missing header to fix 'W=1' warning

Add a missing header otherwise compiler warns about missed prototype:

drivers/xen/xlate_mmu.c:183:5: warning: no previous prototype for 'xen_xlate_unmap_gfn_range?' [-Wmissing-prototypes]
  int xen_xlate_unmap_gfn_range(struct vm_area_struct *vma,
      ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Srikanth Boddepalli <boddepalli.srikanth@gmail.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>