qtdemux: unref simple caps after use

Otherwise we leak a GSTCaps object every time we
use qtdemux on a file with Opus audio

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8879>

subprojects/gst-plugins-good/gst/isomp4/qtdemux.c

@@ -15073,6 +15073,7 @@ qtdemux_parse_trak (GstQTDemux * qtdemux, GNode * trak, guint32 * mvhd_matrix)
             goto corrupt_file;
           }
 
+          gst_caps_unref (entry->caps);
           entry->caps = gst_codec_utils_opus_create_caps (rate, n_channels,
               channel_mapping_family, stream_count, coupled_count,
               channel_mapping);