netsurf.buildsystem: 1.5 -> 1.6

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.)
- directory tree listing: https://gist.github.com/8d2bc8fda08696882cec0fd25ff22ac3