ACPI: scan: Make acpi_walk_dep_device_list()

Because acpi_walk_dep_device_list() is only called by the code in the
file in which it is defined, make it static, drop the export of it
and drop its header from acpi.h.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>