Make QSvgExtraStates' default ctor private


...to make sure it always gets initialized with a proper QSvgDocument.

Pick-to: 6.12
Change-Id: I1fe3a33ded5a402c2b485b8b7c7fece8fda1883d
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>
commit394a3332c8f6652f45cc2f9278362b57fe0b6fd8
parent902515ae8e7fc19cc3b47c9cdb475a902fb4d720
authorRobert Löhning <robert.loehning@qt.io>
committerRobert Löhning <robert.loehning@qt.io>
commit timeMon, 21 Sep 2026 07:24:03 +0000