Mark interface_name() const.

PiperOrigin-RevId: 519844633
diff --git a/quiche/quic/tools/quic_client_base.h b/quiche/quic/tools/quic_client_base.h
index c3ec302..d17503f 100644
--- a/quiche/quic/tools/quic_client_base.h
+++ b/quiche/quic/tools/quic_client_base.h
@@ -297,7 +297,7 @@
     interface_name_ = interface_name;
   }
 
-  std::string interface_name() { return interface_name_; }
+  std::string interface_name() const { return interface_name_; }
 
   void set_server_connection_id_override(
       const QuicConnectionId& connection_id) {