include: xen: add gnttab_query_size struct to public headers

Xen public headers were imported into Zephyr source tree and only used
structs were added (not copied as is). Mow, for refactoring and
improving gnttab driver we need to augment grant table public header
with one more struct. It will be used for reading actual number of
available Xen grant frames.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>