mft: Add a prefix to local definitions of GUIDs
This fixes building with the very latest nightly version of mingw-w64.
In https://github.com/mingw-w64/mingw-w64/commit/7fb7c9f6e401e2bc4393e61a27a68ebccffa1195, the MFVideoFormat_L8 mediatype GUID was added, conflicting with our own definition with the same name.