Add xcb lib if x11extras is detected

In this case we use directly some xcb api outside of the Qt framework
and hence we have to add the xcb library by ourselves.

config/qt.m4

@@ -464,6 +464,7 @@ EOF4
 		    fi
 		    QT5_X11_EXTRAS=`cd $lyx_test_qt_dir; make -s -f $lyx_test_qt_mak DEFINES | grep QT_X11EXTRAS_LIB`
 		    if test -n "$QT5_X11_EXTRAS"; then
+			QT_LIB="$QT_LIB -lxcb"
 			AC_DEFINE(HAVE_QT5_X11_EXTRAS, 1,
 				[Define if you have the Qt5X11Extras module])
 		    fi