acpidump: silent the warning about unaligned uuid

The field comes from the ACPI NFIT table and must be already properly
aligned by BIOS (at least so is written in the spec).

Reviewed by: markj
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44634