adaptive: AbstractAdapatiationLogic: mark getPeriodDuration as override

Fix the warning:

../../modules/demux/adaptive/logic/AbstractAdaptationLogic.h:51:45: warning: 'updateDownloadRate' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
                virtual void                updateDownloadRate     (const ID &, size_t, vlc_tick_t);
                                            ^

(cherry picked from commit e57bd2ac)