cpupower: split unitdir from libdir in Makefile

Improve the installation procedure for the systemd service unit
'cpupower.service', to be more flexible. Some distros install libraries
to /usr/lib64/, but systemd service units have to be installed to
/usr/lib/systemd/system: as a consequence, the installation procedure
should not assume that systemd service units can be installed to
${libdir}/systemd/system ...
Define a dedicated variable ("unitdir") in the Makefile.

Link: https://lore.kernel.org/linux-pm/260b6d79-ab61-43b7-a0eb-813e257bc028@leemhuis.info/T/#m0601940ab439d5cbd288819d2af190ce59e810e6

Fixes: 9c70b779 ("cpupower: add a systemd service to run cpupower")

Link: https://lore.kernel.org/r/20250521211656.65646-1-invernomuto@paranoici.org


Signed-off-by: Francesco Poli (wintermute) <invernomuto@paranoici.org>
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>