smplayer: 18.5.0 -> 18.6.0 (#42674)

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

This update was made based on information from https://repology.org/metapackage/smplayer/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/9qa6s45c82jpkwqaf32i423my6by5ynk-smplayer-18.6.0/bin/smplayer had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/9qa6s45c82jpkwqaf32i423my6by5ynk-smplayer-18.6.0/bin/simple_web_server had a zero exit code or showed the expected version
- 0 of 2 passed binary check by having a zero exit code.
- 0 of 2 passed binary check by having the new version present in output.
- found 18.6.0 with grep in /nix/store/9qa6s45c82jpkwqaf32i423my6by5ynk-smplayer-18.6.0
- directory tree listing: https://gist.github.com/8b3f0826d462c03465088136a4d84512
- du listing: https://gist.github.com/f1d3e833d4b6317c73836b43e52e12fe