libxml2: 2.9.7 -> 2.9.8

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.9.8 in filename of file in /nix/store/cjycf1wx5a5l22a9kwhpnnh2h9i7pahk-libxc-4.0.4