Fix missing using statement

Author: simon-lemay-unity

com.unity.netcode.gameobjects/Tests/Editor/Transports/UnityTransportTests.cs

@@ -1,5 +1,6 @@
 using NUnit.Framework;
 using Unity.Netcode.Transports.UTP;
+using Unity.Networking.Transport;
 using UnityEngine;
 using UnityEngine.TestTools;