mainwindow: gated gdk_wayland_* calls behind version check

[sp1ritCS]

This should ensure that NoteKit will continue to be build on RHEL 8, as this currently fails with:

[   99s] ../mainwindow.cpp: In member function ‘void CMainWindow::SettingCsdUpdate()’:
[   99s] ../mainwindow.cpp:801:5: error: ‘gdk_wayland_window_announce_ssd’ was not declared in this scope
[   99s]      gdk_wayland_window_announce_ssd(window);
[   99s]      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[   99s] ../mainwindow.cpp:801:5: note: suggested alternative: ‘gdk_wayland_window_announce_csd’
[   99s]      gdk_wayland_window_announce_ssd(window);
[   99s]      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[   99s]      gdk_wayland_window_announce_csd