Adding vmlinux to linux kernel 'dev' derivation.

It takes some extra 13MB (and in dev, not out), but allows perf to show kernel
symbols when profiling. I think it is worth it.

In my NixOS, I refer to it in the system derivation, for easy telling to perf
through /run/booted-system/vmlinux:

  system.extraSystemBuilderCmds = ''
    ln -s ${config.boot.kernelPackages.kernel.dev}/vmlinux $out/vmlinux
  '';