ACPI: fix acpi_bus.h build warnings when ACPI is not enabled

introduced in Linux-3.5-rc1 by
66886d6f
(ACPI: Add stubs for (un)register_acpi_bus_type)

Fix header file warnings when CONFIG_ACPI is not enabled:

include/acpi/acpi_bus.h:443:42: warning: 'struct acpi_bus_type' declared inside parameter list
include/acpi/acpi_bus.h:443:42: warning: its scope is only this definition or declaration, which is probably not
include/acpi/acpi_bus.h:444:44: warning: 'struct acpi_bus_type' declared inside parameter list

Signed-off-by: Len Brown <len.brown@intel.com>