Make Attribute copyable again, for subclasses which implement clone()

This fixes compilation of (at least) akonadi-mime.

CCMAIL: dvratil@kde.org

src/core/attribute.h

@@ -175,7 +175,7 @@ public:
 
 protected:
     explicit Attribute() = default;
-    Q_DISABLE_COPY_MOVE(Attribute)
+    Attribute(const Attribute &) = default;
 };
 
 }