Fix warning for -no-feature-textcodec

Change-Id: I7775c8611a532db96f9843c311463a69c2e9ef82
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@qt.io>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -284,6 +284,8 @@ static inline XTextProperty* qstringToXTP(Display *dpy, const QString& s)
         free_prop = false;
 #if QT_CONFIG(textcodec)
     }
+#else
+    Q_UNUSED(dpy);
 #endif
     return &tp;
 }