Include the required headers from wx/custombgwin.h.

Make this header self-contained, i.e. safe to include as the first wxWidgets
header being included from the user code.

Closes #16391.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76947 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775