MINOR: mux-h2/traces: add a missing trace on connection WU with negative inc

The test was performed but no trace emitted, which can complicate certain
diagnostics, so let's just add the trace for this rare case. It may safely
be backported though this is really not important.

src/mux_h2.c

@@ -2604,6 +2604,7 @@ static int h2c_handle_window_update(struct h2c *h2c, struct h2s *h2s)
 		}
 
 		if (h2c->mws >= 0 && h2c->mws + inc < 0) {
+			TRACE_ERROR("conn WINDOW_UPDATE inc<0", H2_EV_RX_FRAME|H2_EV_RX_WU, h2c->conn);
 			error = H2_ERR_FLOW_CONTROL_ERROR;
 			goto conn_err;
 		}