ACPI / tables: Fix DSDT override mechanism

Commit 5ae74f2c (ACPI / tables: Move table override mechanisms to
tables.c) forgot to move the CONFIG_ACPI_CUSTOM_DSDT_FILE inclusion
directive from osl.c to tables.c.  Fix that.

Fixes: 5ae74f2c (ACPI / tables: Move table override mechanisms to tables.c)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Lv Zheng <lv.zheng@intel.com>