Looks like ANT Migration Tool doesn't allow to migrate FLS for standard fields of standard objects.
Metadata API Documentation says here and here that you can retrieve and deploy only Custom field permissions. So it is not possible to migrate Standard Field Permission by ANT Migration Tool since it is using Metadata API which doesn't allow to do that.
Anyway you can use a URL Hack to copy FLS from one sandbox to another
if you have some active confirmation token you can specify it to perform this operation in one click otherwise you will receive message that "The page you submitted was invalid for your session. Please click Save again to confirm your change." and will have just to click save.
The request url is https://test.salesforce.com/_ui/common/config/field/StandardFieldAttributes/e
and parameters are
setupid = CaseFields - Setup Id Entity name
id = BusinessHours - standard field name (or custom field id)
type = Case - standard object name (or custom object id)
save = Save - command to perform save even in GET request
d00eE0000000z6ChIAI = 1 - Set "Visible" checkbox for profile with Id 00eE0000000z6ChIAI
r00eE0000000z6ChIAI = 1 - Set "Readonly" checkbox for profile with Id 00eE0000000z6ChIAI
_CONFIRMATIONTOKEN = Vmp...o - Confirmation token of action, used to perform action in one click (read above)