fixed compiler errors

Author: oleghcp

Code/Editor/Drawers/AssetRefDrawer.cs

@@ -1,4 +1,7 @@
 #if UNITY_2020_1_OR_NEWER
+#if INCLUDE_ADDRESSABLES
+using System.Runtime.CompilerServices;
+#endif
 using System;
 using System.Linq;
 using OlegHcp;

Code/Runtime/OlegHcp/AssetRef.cs

@@ -1,6 +1,7 @@
 #if UNITY_2020_1_OR_NEWER
 #if INCLUDE_ADDRESSABLES
 using UnityEngine.AddressableAssets;
+using System.Runtime.CompilerServices;
 #endif
 using System;
 using UnityEngine;

Code/Runtime/System/Runtime/CompilerServices/SwitchExpressionException.cs

@@ -1,4 +1,5 @@
-namespace System.Runtime.CompilerServices
+#if !UNITY_2021_2_OR_NEWER
+namespace System.Runtime.CompilerServices
 {
     public sealed class SwitchExpressionException : InvalidOperationException
     {
@@ -38,3 +39,4 @@ private static string GetValueMessage(object value)
         }
     }
 }
+#endif