cpufrequtils: add mainProgram `cpufreq-set`

Signed-off-by: Christina Sørensen <[e-mail]>

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

···
     homepage = "http://ftp.be.debian.org/pub/linux/utils/kernel/cpufreq/cpufrequtils.html";
     license = licenses.gpl2Only;
     platforms = [ "x86_64-linux" ];
+    mainProgram = "cpufreq-set";
   };
 }