irqchip/mbigen: Fix compile warning when CONFIG_ACPI is disabled

Fix the following compile warning:

  drivers/irqchip/irq-mbigen.c:372:36: warning: ‘mbigen_acpi_match’ defined but not used [-Wunused-const-variable=]
   static const struct acpi_device_id mbigen_acpi_match[] = {

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210519050455.1693953-1-yangyingliang@huawei.com