Fix ODR violation with MouseClick plugin

```
/var/tmp/portage/kde-plasma/kwin-6.0.49.9999/work/kwin-6.0.49.9999/src/input_event.h:21:7: error: type ‘struct MouseEvent’ violates the C++ One Definition Rule [-Werror=odr]
   21 | class MouseEvent : public QMouseEvent
      |       ^
/var/tmp/portage/kde-plasma/kwin-6.0.49.9999/work/kwin-6.0.49.9999/src/plugins/mouseclick/mouseclick.h:27:7: note: a type with different bases is defined in another translation unit
   27 | class MouseEvent
      |       ^
lto1: some warnings being treated as errors
```

Bug: https://bugs.gentoo.org/921558


Signed-off-by: Sam James <sam@gentoo.org>


(cherry picked from commit 53a61dfa)