Reset WaylandServer::s_self in destructor

When using KWIN_SINGLETON macros, you still need to reset s_self
manually in the destructor.

src/wayland_server.cpp

@@ -204,6 +204,7 @@ WaylandServer::WaylandServer(QObject *parent)
 WaylandServer::~WaylandServer()
 {
     destroyInputMethodConnection();
+    s_self = nullptr;
 }
 
 KWaylandServer::ClientConnection *WaylandServer::xWaylandConnection() const