grl-media-source: signal finished metadata() operations
grl-media-source: signal finished metadata() operations

We were previously leaking metadata() operations by never signalling
them as "finished".

Signed-off-by: Lionel Landwerlin <9463032a5b99ffad10f36631678a833369a311b5@linux.intel.com>

kyoushuu/grilo,MathieuDuponchelle/grilo,jasuarez/grilo,jasuarez/grilo,GNOME/grilo,grilofw/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,GNOME/grilo,grilofw/grilo,kyoushuu/grilo,jasuarez/grilo,kyoushuu/grilo,grilofw/grilo,MathieuDuponchelle/grilo