Ensure that wxGenericListCtrl always has wx[HV]SCROLL styles.

Ensure that wxGenericListCtrl always has wx[HV]SCROLL styles.

Although these styles are unconditionally added when creating the window they
could be reset by calling SetWindowStyleFlag() later and this in fact happened
when changing control mode from wxListbook.

As it doesn't make sense to have wxListCtrl without these styles, also add
them unconditionally in overridden SetWindowStyleFlag() as well.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64882 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775