auto-cpufreq: actually run as deamon

The upstream service definition uses the (undocumented) --daemon
option, which causes the process to stay running and not exit
immediately.

nixos/modules/services/hardware/auto-cpufreq.nix

@@ -37,7 +37,7 @@ in {
 
         serviceConfig.ExecStart = [
           ""
-          "${lib.getExe pkgs.auto-cpufreq} --config ${cfgFile}"
+          "${lib.getExe pkgs.auto-cpufreq} --daemon --config ${cfgFile}"
         ];
       };
     };