ACPICA: Update check for setting the ANOBJ_IS_EXTERNAL flag.

The ANOBJ_IS_EXTERNAL flag is only used by an ACPICA utilities - iASL.

No functional change for the Linux kernel should results from applying
this patch, but it helps to reduce source code differences between
the kernel and ACPICA upstream.

[rjw: Subject and changelog]
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>