fix

Wrapping reference to namespace that only exists when in unified mode.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -8,7 +8,9 @@
 #endif
 using Unity.Netcode.Components;
 using Unity.Netcode.Runtime;
+#if UNIFIED_NETCOE
 using Unity.Netcode.Unified;
+#endif
 using UnityEngine;
 #if UNITY_EDITOR
 using UnityEditor;