Fix build with QT_NO_CLIPBOARD

Change-Id: I138a29e1099e691770b9e3e094a108fc8c52f8cb
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/widgets/dialogs/qmessagebox.cpp

@@ -133,10 +133,14 @@ public:
 
     bool copy()
     {
+#ifdef QT_NO_CLIPBOARD
+        return false;
+#else
         if (!copyAvailable)
             return false;
         textEdit->copy();
         return true;
+#endif
     }
 
 private slots: