Add missing \since to QList<T>::swapItemsAt()

Amends 7f4d0405b4.

Change-Id: I4de38428ea4a0e448e2930d19d94821884f7331e
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qlist.cpp

@@ -971,6 +971,7 @@ void **QListData::erase(void **xi)
 */
 
 /*! \fn template <class T> void QList<T>::swapItemsAt(int i, int j)
+    \since 5.13
 
     Exchange the item at index position \a i with the item at index
     position \a j. This function assumes that both \a i and \a j are