gio docs: add win32 "stubs" hack
Submitted by Allison (desrt)
Link to original bug (#746119)
Description
Add a hack to ensure that we can extract the type hierarchy and property information for GWin32{Input,Output}Stream, even for builds on UNIX systems.
This will allow us to have this information in the online manual, and in the html included in the tarballs.
It also means that we can drop the hardcoding of our .types file, switching over to --rebuild-types instead.