dashdemux: use new gst_mpd_client_get_next_fragment_duration API
dashdemux: use new gst_mpd_client_get_next_fragment_duration API

This fixes build that has been broken by commit
fb9aeac6552021b176a4c4bd07265e02a0b70e0f.

gst_mpd_client_get_target_duration has been removed, and
gst_mpd_client_get_next_fragment_duration should be used instead.

Orange-OpenSource/gstdashdemux,Orange-OpenSource/gstdashdemux,Orange-OpenSource/gstdashdemux