QAbstractAnimation: add missing forward-declaration

QUnifiedTimer is declared as a friend using an unqualified name that
hasn't been forward-declared before. In this situation, the friend
declaration declares the name. This is subtle, and while it's ok here,
in other situations it can actually lead to two different names being
declared.

Fix by adding the missing forward declaration.

Pick-to: 6.3 6.2 5.15
Change-Id: Ie9f5165578f1d88c6bee1afa23ef43847755a0eb
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/animation/qabstractanimation.h

@@ -49,6 +49,7 @@ QT_BEGIN_NAMESPACE
 class QAnimationGroup;
 class QSequentialAnimationGroup;
 class QAnimationDriver;
+class QUnifiedTimer;
 
 class QAbstractAnimationPrivate;
 class Q_CORE_EXPORT QAbstractAnimation : public QObject