Fix CodeQL violation

Author: commonsensesoftware

src/AspNetCore/WebApi/src/Asp.Versioning.Http/Builder/EndpointBuilderFinalizer.cs

@@ -278,10 +278,10 @@ private record struct ApiVersionBuckets(
         IReadOnlyList<ApiVersion> Advertised,
         IReadOnlyList<ApiVersion> AdvertisedDeprecated )
     {
-        internal bool AreEmpty = Mapped.Count == 0
-                                 && Supported.Count == 0
-                                 && Deprecated.Count == 0
-                                 && Advertised.Count == 0
-                                 && AdvertisedDeprecated.Count == 0;
+        internal readonly bool AreEmpty = Mapped.Count == 0
+                                          && Supported.Count == 0
+                                          && Deprecated.Count == 0
+                                          && Advertised.Count == 0
+                                          && AdvertisedDeprecated.Count == 0;
     }
 }