ACPI / tables: Add an ifdef around amlcode and dsdt_amlcode

Clang warns:

drivers/acpi/tables.c:715:14: warning: unused variable 'amlcode'
[-Wunused-variable]
static void *amlcode __attribute__ ((weakref("AmlCode")));
             ^
drivers/acpi/tables.c:716:14: warning: unused variable 'dsdt_amlcode'
[-Wunused-variable]
static void *dsdt_amlcode __attribute__ ((weakref("dsdt_aml_code")));
             ^
2 warnings generated.

The only uses of these variables are hiddem behind CONFIG_ACPI_CUSTOM_DSDT
so do the same thing here.

Fixes: 82e4eb4e (ACPI / tables: add DSDT AmlCode new declaration name support)
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>