Fix missing Ui namespace

Author: EricKotato

Telegram/SourceFiles/boxes/confirm_box.h

@@ -124,7 +124,7 @@ class MaxInviteBox : public Ui::BoxContent, private base::Subscriber {
 
 };
 
-class ConvertToSupergroupBox : public BoxContent, public RPCSender {
+class ConvertToSupergroupBox : public Ui::BoxContent, public RPCSender {
 public:
 	ConvertToSupergroupBox(QWidget*, not_null<ChatData*> chat);