fix(lxml): Add missing zlib dependency

Somewhere between nixpkgs 693bc46d169f5af9c992095736e82c3488bf7dbb and
52ec9ac3b12395ad677e8b62106f0b98c1f8569d, lxml stopped building
because it could no longer use zlib as pulled in by one of its
dependencies (perhaps Python itself?)

overrides/default.nix

@@ -1474,7 +1474,7 @@ lib.composeManyExtensions [
             old.nativeBuildInputs or [ ]
             ++ [ pkg-config libxml2.dev libxslt.dev ]
             ++ lib.optionals stdenv.isDarwin [ xcodebuild ];
-          buildInputs = old.buildInputs or [ ] ++ [ pkgs.libxml2 pkgs.libxslt ];
+          buildInputs = old.buildInputs or [ ] ++ [ pkgs.libxml2 pkgs.libxslt pkgs.zlib ];
         }
       );