Fix 'unused parameter' warnings

Summary:
The warnings:

```
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgforeign_v2_interface.cpp: In static member function ‘static void
KWayland::Server::XdgExporterUnstableV2Interface::Private::destroyCallback(wl_client*, wl_resource*)’:
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgforeign_v2_interface.cpp:95:95: warning: unused parameter ‘resource’
[-Wunused-parameter]
 void XdgExporterUnstableV2Interface::Private::destroyCallback(wl_client *client, wl_resource *resource)
                                                                                  ~~~~~~~~~~~~~^~~~~~~~
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgforeign_v2_interface.cpp: In static member function ‘static void
KWayland::Server::XdgImporterUnstableV2Interface::Private::destroyCallback(wl_client*, wl_resource*)’:
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgforeign_v2_interface.cpp:237:95: warning: unused parameter ‘resource’
[-Wunused-parameter]
 void XdgImporterUnstableV2Interface::Private::destroyCallback(wl_client *client, wl_resource *resource)
                                                                                  ~~~~~~~~~~~~~^~~~~~~~
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgforeign_v2_interface.cpp: In static member function ‘static void
KWayland::Server::XdgImportedUnstableV2Interface::Private::setParentOfCallback(wl_client*, wl_resource*, wl_resource*)’:
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgforeign_v2_interface.cpp:439:78: warning: unused parameter ‘client’
[-Wunused-parameter]
 void XdgImportedUnstableV2Interface::Private::setParentOfCallback(wl_client *client, wl_resource *resource, wl_resource *
surface)
                                                                   ~~~~~~~~~~~^~~~~~

/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgoutput_interface.cpp: In constructor
‘KWayland::Server::XdgOutputManagerInterface::XdgOutputManagerInterface(KWayland::Server::Display*, QObject*)’:
/home/vlad/KDE/src/frameworks/kwayland/src/server/xdgoutput_interface.cpp:88:81: warning: unused parameter ‘parent’
[-Wunused-parameter]
 XdgOutputManagerInterface::XdgOutputManagerInterface(Display *display, QObject *parent)
                                                                        ~~~~~~~~~^~~~~~
```

Test Plan: No more 'unused parameter' warnings.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D13501