mkv: Restore the priority of the mkv module

Most of the time the module was still selected due to extension lookup.
It is also reported that avformat still is unstable when seeking mkv's

modules/demux/mkv/mkv.cpp

@@ -42,7 +42,7 @@ static void Close( vlc_object_t * );
 vlc_module_begin ()
     set_shortname( "Matroska" )
     set_description( N_("Matroska stream demuxer" ) )
-    set_capability( "demux", 0 )
+    set_capability( "demux", 50 )
     set_callbacks( Open, Close )
     set_category( CAT_INPUT )
     set_subcategory( SUBCAT_INPUT_DEMUX )