Switch to libossp-uuid, to get UUIDs on Windows

This was only used to avoid a necessary check in collab builds on
Windows. @jtanx, who does the Windows binaries, says collab currently
isn’t building on Windows anyway.
I'm looking at the merged changes but don't see any hint to UUID_LEN_STR. It might make sense to add a #define in the area to avoid hunting the code in the future.

    #define  FF_UUID_STRING_LEN 33 

I'd love to also define a fixed length string type ff_uuid_string_t, but there are some edge cases for that which would call for a wider policy across the code.