Forward declaring as a struct a symbol previously defined as a typedef results
in an error when using MinGW-w64 4.8.1, so forward declare the struct itself
instead.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74883 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775