xcb: make beep work

Adding xcb_flush after xcb_bell makes it work, no need to move the
mouse.

Fixes: QTBUG-75617
Change-Id: Ieeb47468bf31cfa6fcf2d48da56d54b9e6eac6fe
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -550,6 +550,7 @@ void QXcbIntegration::beep() const
         return;
     xcb_connection_t *connection = static_cast<QXcbScreen *>(screen)->xcb_connection();
     xcb_bell(connection, 0);
+    xcb_flush(connection);
 }
 
 bool QXcbIntegration::nativePaintingEnabled() const