cpupower: prepare for 6.16-rc2

pkgs/os-specific/linux/cpupower/default.nix

···
     doc = "share/doc/cpupower";
     conf = "etc";
     bash_completion_ = "share/bash-completion/completions";
+    unit = "lib/systemd/system";
   };
 
   enableParallelBuilding = true;