Describe the bug
Various Qt applications (smplayer, qjackctl etc) use Windows style regardless of what style is selected in KDE System Settings.
When I try to pass style via command line it doesn't work either.

qjackctl -style Breeze
QApplication: invalid style override 'Breeze' passed, ignoring it.
        Available styles: Breeze, Windows, Fusion

To Reproduce
Steps to reproduce the behavior:

1. Select any style (Breeze, Fusion) in KDE System Settings
2. Launch Qt application (smplayer for example)
3. Application uses Windows style

Expected behavior
Qt applications should use style selected in KDE System Settings

Screenshots

Metadata

- system: `"x86_64-linux"`
 - host os: `Linux 5.7.19, NixOS, 21.03pre243690.441a7da8080 (Okapi)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.7`
 - channels(root): `"nixos-21.03pre243690.441a7da8080"`
 - channels(alapshin): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`