MINOR: quic: redirect app_ops snd_buf through mux

This change is required to be able to use multiple app_ops layer on top
of QUIC. The stream-interface will now call the mux snd_buf which is
just a proxy to the app_ops snd_buf function.

The architecture may be simplified in the structure to install the
app_ops on the stream_interface and avoid the detour via the mux layer
on the sending path.
diff --git a/src/h3.c b/src/h3.c
index 352b8c2..9be2a5b 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -820,5 +820,6 @@
 	.init        = h3_init,
 	.attach_ruqs = h3_attach_ruqs,
 	.decode_qcs  = h3_decode_qcs,
+	.snd_buf     = h3_snd_buf,
 	.finalize    = h3_finalize,
 };