iproute_mptcp: Fix the build

My last iproute2 update (247aed0528) broke the build due to the new
patch [0].

Reported-by: Sergei Trofimovich <slyich@gmail.com>

[0]: https://github.com/NixOS/nixpkgs/pull/136701#issuecomment-924920542

pkgs/os-specific/linux/iproute/mptcp.nix

@@ -11,12 +11,13 @@ iproute2.overrideAttrs (oa: rec {
     sha256 = "07fihvwlaj0ng8s8sxqhd0a9h1narcnp4ibk88km9cpsd32xv4q3";
   };
 
-  preConfigure = ''
-    # Don't try to create /var/lib/arpd:
-    sed -e '/ARPDDIR/d' -i Makefile
+  preConfigure = oa.preConfigure + ''
     patchShebangs configure
   '';
 
+  # We override "patches" to never apply any iproute2 patches:
+  patches = [ ];
+
   meta = with lib; {
     homepage = "https://github.com/multipath-tcp/iproute-mptcp";
     description = "IP-Route extensions for MultiPath TCP";