xen/grant-table: Export gnttab_{alloc|free}_pages as GPL

Only gnttab_{alloc|free}_pages are exported as EXPORT_SYMBOL
while all the rest are exported as EXPORT_SYMBOL_GPL, thus
effectively making it not possible for non-GPL driver modules
to use grant table module. Export gnttab_{alloc|free}_pages as
EXPORT_SYMBOL_GPL so all the exports are aligned.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>