Fix qio_file_path_for_fd so it works in cygwin

`qio_file_path_for_fd` has three paths that are selected via `#ifdef`s, cygwin was
hitting the fall-back case that always returned "unknown" as the file path.

cygwin can actually use the same code as linux for this operation, so I've
added a check for `__CYGWIN__` to that `#if`.