ACPI / DPTF: remove header search path to the parent directory

It is too much to add extra header search path for all files in
drivers/acpi/dptf/.

Fix up one C file, and remove the header search path.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>