adaptive: Segment: mark debug as override

Fix warning:

../../modules/demux/dash/mp4/../mpd/../../adaptive/playlist/Segment.h:103:30: warning: 'debug' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
                virtual void debug(vlc_object_t *,int = 0) const;

(cherry picked from commit f65953ef)