MINOR: quic: Add the QUIC connection to mux traces

This should help for debugging purpose.

Should be backported to 2.6
diff --git a/src/mux_quic.c b/src/mux_quic.c
index a2814c7..e29fa1c 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -2367,6 +2367,8 @@
 
 	if (src->verbosity > QMUX_VERB_CLEAN) {
 		chunk_appendf(&trace_buf, " : qcc=%p(F)", qcc);
+		if (qcc->conn->handle.qc)
+			chunk_appendf(&trace_buf, " qc=%p", qcc->conn->handle.qc);
 
 		if (qcs)
 			chunk_appendf(&trace_buf, " qcs=%p .id=%llu .st=%s",