mpc-qt: 17.11 -> 18.03

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 18.03 with grep in /nix/store/b23s2dr5x7zrhp4vgbh8nf2gajp8a44b-mpc-qt-18.03
- directory tree listing: https://gist.github.com/4792f038cb0905bcd3405765bb63b792