tools/power/acpi: Add .gitignore file

Add a .gitignore file so that git commands do not pick up the resulting
binaries and directories.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>