Utils: Make QtColorButton's d_ptr private, again

Amends: 03aed5c5a6

Change-Id: Ic9e99d4e63bc54f7931cefb084a7f5a215dbc109
Reviewed-by: Michael Winkelmann <michael.winkelmann@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/qtcolorbutton.h

@@ -68,7 +68,7 @@ protected:
     void dragLeaveEvent(QDragLeaveEvent *event) override;
     void dropEvent(QDropEvent *event) override;
 #endif
-
+private:
     class QtColorButtonPrivate *d_ptr;
     friend class QtColorButtonPrivate;
 };