fix file not found cecilx exception

Assets/Mirror/Editor/Weaver/EntryPointILPostProcessor/ILPostProcessorHook.cs

@@ -5,6 +5,9 @@
 // https://forum.unity.com/threads/how-does-unity-do-codegen-and-why-cant-i-do-it-myself.853867/#post-5646937
 using System.IO;
 using System.Linq;
+// to use Mono.CecilX here, we need to 'override references' in the
+// Unity.Mirror.CodeGen assembly definition file in the Editor, and add CecilX.
+// otherwise we get a reflection exception with 'file not found: CecilX'.
 using Mono.CecilX;
 using Unity.CompilationPipeline.Common.ILPostProcessing;
 // IMPORTANT: 'using UnityEngine' does not work in here.

Assets/Mirror/Editor/Weaver/Unity.Mirror.CodeGen.asmdef

@@ -9,8 +9,10 @@
     ],
     "excludePlatforms": [],
     "allowUnsafeCode": true,
-    "overrideReferences": false,
-    "precompiledReferences": [],
+    "overrideReferences": true,
+    "precompiledReferences": [
+        "Mono.CecilX.dll"
+    ],
     "autoReferenced": false,
     "defineConstraints": [],
     "versionDefines": [],