dlib: 19.9 -> 19.10

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 19.10 with grep in /nix/store/3h9m66pcyc428v616z3ls200aidd12wz-dlib-19.10
- directory tree listing: https://gist.github.com/7c3e568146e46ccdaed28be2916bd0a9