WMainMenuBar: Fix clazy error

[uklotzde]

#3844 (comment)

[ronso0]

expected with --fullScreen and/or fullscreen option set:

• launchscreen
• skin
• menubar

results are strange, but the same in plain main:

• with neither --fullScreen nor fullscreen option:

  • starts in window mode
  • launchscreen
  • skin
  • style

• with the fullscreen option (optional --fullScreen) it works nicely:

  • (other app focused, Mixxx window is pushed to the foreground)
  • launchscreen
  • skin
  • style menubar

• with --fullScreen only it gets weird (while it works fine in 2.3)

  • starts in window mode
  • (Mixxx window is pushed to the foreground)
  • launchscreen
  • (Alt+Tab to other app)
  • (Mixxx window is pushed to the foreground)
  • unstyled menubar
  • skin
  • style menubarmenubar

[ronso0]

so, yeah, the change keeps the status quo.
assumption: something related to CoreServices broke the --f arg

same change applied to 2.3 works fine.

Thanks, LGTM