ACPI: blacklist: fix clang warning for unused DMI table

When CONFIG_DMI is disabled, we only have a tentative declaration,
which causes a warning from clang:

drivers/acpi/blacklist.c:20:35: error: tentative array definition assumed to have one element [-Werror]
static const struct dmi_system_id acpi_rev_dmi_table[] __initconst;

As the variable is not actually used here, hide it entirely
in an #ifdef to shut up the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>