gns3Packages.server{Stable,Preview}: Fix the build

I initially thought python3Packages.yarl wouldn't build but then
realized it was actually due to my overwrite.

pkgs/applications/networking/gns3/server.nix

···
       }));
   yarl = (stdenv.lib.overrideDerivation pythonPackages.yarl
     (oldAttrs:
-      { propagatedBuildInputs = [ multidict_3_1_3 ]; }));
+      { propagatedBuildInputs = [ multidict_3_1_3 pythonPackages.idna ]; }));
   aiohttp = (stdenv.lib.overrideDerivation pythonPackages.aiohttp
     (oldAttrs:
       rec {