grant_table: include mm.h in xen/grant_table.h

grant_table: include mm.h in xen/grant_table.h

While re-ordering the include alphabetically in arch/arm/domain.c, I got
a complitation error because grant_table.h is using gfn_t before been
defined:

In file included from domain.c:14:0:
xen/xen/include/xen/grant_table.h:153:29: error: unknown type name \91gfn_t\92
                             gfn_t *gfn, uint16_t *status);
                             ^

Fix it by including xen/mm.h in it.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>