LinuxKPI: add guid_t for ACPI consumers.

Add a placeholder struct for guid_t which is needed by ACPI consumers
in at least one wireless driver.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30439

(cherry picked from commit cae1683120e2bf8e39901d29a17c473401653985)