qsynth: 0.5.0 -> 0.5.1

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

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

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/v9m2gbw7l68qb4m5milqc8d2wfrafx97-qsynth-0.5.1/bin/qsynth had a zero exit code or showed the expected version
- 0 of 1 passed binary check by having a zero exit code.
- 0 of 1 passed binary check by having the new version present in output.
- found 0.5.1 with grep in /nix/store/v9m2gbw7l68qb4m5milqc8d2wfrafx97-qsynth-0.5.1
- directory tree listing: https://gist.github.com/f7434d80382444c8ae9ba52dd07e27a7
- du listing: https://gist.github.com/1c209e5c3a841d8d8e1d3257c2dad992