ACPICA: Conditionally define a local variable that is used for debug only.

This patch improves the relationship between the opcode debugging
information and CONFIG_ACPI_DEBUG enablement.

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