Support Struct for DynamicLinqTypeAttribute (#579)

Author: StefH

src/System.Linq.Dynamic.Core/CustomTypeProviders/DynamicLinqTypeAttribute.cs

@@ -3,8 +3,8 @@
     /// <summary>
     /// Indicates to Dynamic Linq to consider the Type as a valid dynamic linq type.
     /// </summary>
-    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Enum, AllowMultiple = false, Inherited = false)]
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum, AllowMultiple = false, Inherited = false)]
     public sealed class DynamicLinqTypeAttribute : Attribute
     {
     }
-}
+}