json_c: 0.13 -> 0.13.1

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

This update was made based on information from https://repology.org/metapackage/json-c/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 0.13.1 with grep in /nix/store/3ckha7sbyy0la7qswa3b01n3wgnccqsf-json-c-0.13.1
- directory tree listing: https://gist.github.com/cad3ba1104556e76a7088a0fd5427df9