Fix compile failure window variable doesn't exist

Summary:
Error was:
```
/home/carl/kdesrc/kde/workspace/kwin/plugins/platforms/x11/windowed/x11windowed_output.cpp: In member function ‘void KWin::X11WindowedOutput::initXInputForWindow()’:
/home/carl/kdesrc/kde/workspace/kwin/plugins/platforms/x11/windowed/x11windowed_output.cpp:134:14: error: invalid use of non-static member function ‘xcb_window_t KWin::X11WindowedOutput::window() const’
  134 |     Q_UNUSED(window)
      |              ^~~~~~
In file included from /home/carl/kdesrc/kde/workspace/kwin/plugins/platforms/x11/windowed/x11windowed_output.cpp:20:
/home/carl/kdesrc/kde/workspace/kwin/plugins/platforms/x11/windowed/x11windowed_output.h:50:18: note: declared here
   50 |     xcb_window_t window() const {
      |
```

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23957