y2038: linux: Add __USE_TIME_BITS64 support for struct timex

The __USE_TIME_BITS64 is not defined internally yet.

Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>