cpupowerutils: use kernel version-derived version string

As cpupowerutils is intended to be included into the kernel sources,
use the kernel versioning instead of a custom version.

The script utils/version-gen.sh is largely based on the script already
found in tools/perf/util/PERF-VERSION-GEN .

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>