xen: mm: include <xen/xen-ops.h> for missing declarations

Include <xen/xen-ops.h> for xen_{create,destroy}_contigous_region
call declarations. Fixes the following sparse warnings:

arch/arm/xen/mm.c:119:5: warning: symbol 'xen_create_contiguous_region' was not declared. Should it be static?
arch/arm/xen/mm.c:131:6: warning: symbol 'xen_destroy_contiguous_region' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>