linux_mptcp: makes linux_mptcp.override works

I needed to override some parameters because of an error I had:
"Error: modDirVersion specified in the Nix expression is wrong, it should be: 4.9.60+"

but the following override would not be taken into account
  pkg.override ({
    modDirVersion="4.9.60+";
    src=pkgs.lib.cleanSource /home/teto/mptcp;
  })

because the override would be overriden by the nixpkgs parameters
because of concatenation order:
https://nixos.org/nix/manual/#sec-language-operators