smplayer: 18.3.0 -> 18.4.0

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

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

These checks were done:

- built on NixOS
- ran ‘/nix/store/5vy4663v65r0ks1d1jcy0p24m2lk0zmh-smplayer-18.4.0/bin/smplayer -h’ got 0 exit code
- ran ‘/nix/store/5vy4663v65r0ks1d1jcy0p24m2lk0zmh-smplayer-18.4.0/bin/smplayer --help’ got 0 exit code
- found 18.4.0 with grep in /nix/store/5vy4663v65r0ks1d1jcy0p24m2lk0zmh-smplayer-18.4.0
- directory tree listing: https://gist.github.com/25ca7c094ad35c4c5ed4c2c33dfb9be2