Ensure that wxAuiNotebook::SetArtProvider() always does set it.

It used to only set the provider if the height of the tabs defined by the new
provider was different from the one used by the old one, otherwise the call
was optimized away. Fix this by explicitly setting the art provider for all
tabs in SetArtProvider() itself if UpdateTabCtrlHeight() didn't do it.

Closes #9738.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65068 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775