qt: workaround for `undefined reference to `qt_version_tag'` error

KF6WindowSystem requires Qt6Gui, but not Qt6Core.
And Qt6Gui does not require Qt6Core.

The symbol `qt_version_tag` is available in Qt6Core.

modules/gui/qt/Makefile.am

@@ -623,7 +623,7 @@ if HAVE_KF6WINDOWSYSTEM
 libqt_plugin_la_CPPFLAGS += -DQT_HAS_KWINDOWSYSTEM_PLUGIN
 
 libqt_kwindowsystem_plugin_la_CXXFLAGS = $(AM_CXXFLAGS) $(KF6WINDOWSYSTEM_CFLAGS)
-libqt_kwindowsystem_plugin_la_LIBADD =  $(KF6WINDOWSYSTEM_LIBS)
+libqt_kwindowsystem_plugin_la_LIBADD =  $(KF6WINDOWSYSTEM_LIBS) $(QT_LIBS)
 libqt_kwindowsystem_plugin_la_SOURCES = \
 	maininterface/kwindowsystem_module.cpp