libwebsockets: 2.4.2 -> 3.0.0

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

This update was made based on information from https://repology.org/metapackage/libwebsockets/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 3.0.0 with grep in /nix/store/m8jcyxq9zxn19vzwmvy8jld4kmjcwnmj-libwebsockets-3.0.0
- directory tree listing: https://gist.github.com/3fda3b4331b75bec575d1a71ba21e84a