libbson: 1.9.3 -> 1.9.4

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/libbson/versions.

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 1.9.4 with grep in /nix/store/120cz3gc3s0hasql1p3zc3sgjzvn4753-libbson-1.9.4
- directory tree listing: https://gist.github.com/da41b1208cf5cb999e6de11ad1fb2fc6