ACPI / APEI: Get rid of NULL_UUID_LE constant

This is a missed part of the commit 5b53696a
("ACPI / APEI: Switch to use new generic UUID API"), i.e.
replacing old definition with a global constant variable.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>