oggstream: consider all opus packets as "keyframes"

This lets oggdemux determine they are not delta units, and removes
spurious per packet warnings about being unable to determine the
packet's keyframeness.

ext/ogg/gstoggstream.c

@@ -2467,7 +2467,7 @@ const GstOggMap mappers[] = {
     granulepos_to_granule_opus,
     granule_to_granulepos_default,
     NULL,
-    NULL,
+    is_packet_keyframe_true,
     is_header_opus,
     packet_duration_opus,
     NULL,