ACPI: tools: fix compilation error

When ACPI tools are compiled, the following error is showed:

   $ cd tools/power/acpi
   $ make
     DESCEND tools/acpidbg
     MKDIR    include
     CP       include
     CC       tools/acpidbg/acpidbg.o
   In file included from /home/linux/tools/power/acpi/include/acpi/platform/acenv.h:152,
                    from /home/linux/tools/power/acpi/include/acpi/acpi.h:22,
                    from acpidbg.c:9:
   /home/linux/tools/power/acpi/include/acpi/platform/acgcc.h:25:10: fatal error: linux/stdarg.h: No such file or directory
      29 | #include <linux/stdarg.h>
         |          ^~~~~~~~~~~~~~~~
   compilation terminated.

Use the ACPICA logic: just identify when it is used inside the kernel
or by an ACPI tool.

Fixes: c0891ac1 ("isystem: ship and use stdarg.h")
Signed-off-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>