platform/x86: wmi: make GUID block packed

The `guid_block` struct is overlaid onto a buffer
coming from the _WDG ACPI object of the device.
For this reason mark the struct packed and add
assertions about sizes.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Link: https://lore.kernel.org/r/20210904175450.156801-16-pobrn@protonmail.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>