seafile-client: 6.1.5 -> 6.1.6

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.)
- found 6.1.6 with grep in /nix/store/304wh7gzixm9as2wa2b619jcimn0837k-seafile-client-6.1.6
- found 6.1.6 in filename of file in /nix/store/304wh7gzixm9as2wa2b619jcimn0837k-seafile-client-6.1.6