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.