lib3270: mark broken on darwin

pkgs/by-name/li/lib3270/package.nix

···
     homepage = "https://github.com/PerryWerneck/lib3270";
     license = licenses.lgpl3Plus;
     maintainers = [ maintainers.vifino ];
+    broken = stdenv.hostPlatform.isDarwin;
   };
 }