tools/power/acpi: fix compilation error

If we compile tools/acpi target in the top source directory, we'd get a
compilation error showing as bellow:

	# make tools/acpi
	  DESCEND  power/acpi
	  DESCEND  tools/acpidbg
	  CC       tools/acpidbg/acpidbg.o
	Assembler messages:
	Fatal error: can't create /home/lzy/kernel-upstream/power/acpi/\
			tools/acpidbg/acpidbg.o: No such file or directory
	../../Makefile.rules:26: recipe for target '/home/lzy/kernel-upstream/\
			power/acpi/tools/acpidbg/acpidbg.o' failed
	make[3]: *** [/home/lzy/kernel-upstream//power/acpi/tools/acpidbg/\
			acpidbg.o] Error 1
	Makefile:19: recipe for target 'acpidbg' failed
	make[2]: *** [acpidbg] Error 2
	Makefile:54: recipe for target 'acpi' failed
	make[1]: *** [acpi] Error 2
	Makefile:1607: recipe for target 'tools/acpi' failed
	make: *** [tools/acpi] Error 2

Fixes: d5a4b1a5 ("tools/power/acpi: Remove direct kernel source include reference")
Signed-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>