ACPI: add acpi_get_cpu_uid() stub helper

ACPI: add acpi_get_cpu_uid() stub helper

When ACPI is disabled, x86 Xen support fails to build:

arch/x86/xen/enlighten_hvm.c: In function 'xen_cpu_up_prepare_hvm':
arch/x86/xen/enlighten_hvm.c:165:13: error: implicit declaration of function 'acpi_get_cpu_uid' [-Wimplicit-function-declaration]
  165 |         if (acpi_get_cpu_uid(cpu, &cpu_uid) == 0)
      |             ^~~~~~~~~~~~~~~~

Add a trivial stub that can be used in place of the real function.

Fixes: f652d0a4e13c ("ACPI: Centralize acpi_get_cpu_uid() declaration in include/linux/acpi.h")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Chengwen Feng <fengchengwen@huawei.com>
Link: https://patch.msgid.link/20260413070132.3828606-1-arnd@kernel.org
Signed-off-by: Rafael J. Wysocki <rjw@rjwysocki.net>