cpupower: do not write DESTDIR to cpupower.service

Fix the use of DESTDIR variable in the Makefile, as far as the
installation of the systemd service unit 'cpupower.service' is
concerned.

This was caused by a misunderstanding about the purpose of the DESTDIR
variable in the Makefile, which is instead meant to support staged
installations: its value should not end up into installed file contents.

Link: https://lore.kernel.org/linux-pm/20250509002206.bd2519ba52035d47c3c32aa6@paranoici.org/T/#mfbb938f9c0d5a21173acb92a061eb9205fd0abfe
Link: https://www.gnu.org/prep/standards/html_node/DESTDIR.html

Link: https://lore.kernel.org/r/20250513163937.61062-3-invernomuto@paranoici.org


Fixes: 9c70b779 ("cpupower: add a systemd service to run cpupower")
Signed-off-by: Francesco Poli (wintermute) <invernomuto@paranoici.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>