ACPICA: acpi_madt_oem_data: Fix flexible array member definition

ACPICA commit e7f6d8c1b7f79eb4b9b07f1bc09c549a2acbd6e8

Use ACPI_FLEX_ARRAY() helper to define flexible array member alone in a
struct. Fixes issue #812.

No binary changes appear in the .text nor .data sections.

Link: https://github.com/acpica/acpica/commit/e7f6d8c1


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>