As part of refactoring out a lot of the switch_* namespace stubs
that don't do anything other than call the underlying apr_* function
it's going to be necessary to include the apr headers directly.

Before the modules could not call the apr functions directly except
via the stubs so, lets make the headers available.

On the whole this should make it possible to delete a lot of the
switch_* namespace functions, which are not being maintained, and
directly call the underlying apr api.