fix build error

Author: eugene-doobu

nekoyume/Assets/_Scripts/Blockchain/BlockManager.cs

@@ -13,6 +13,10 @@
 using Nekoyume.Action;
 using Nekoyume.Model.State;
 using Nekoyume.TableData;
+#if UNITY_ANDROID
+using UnityEngine;
+using UnityEngine.Networking;
+#endif
 
 namespace Nekoyume.Blockchain
 {