Move JsonIgnore specification to interface

osu.Game/Database/IHasPrimaryKey.cs

@@ -2,11 +2,13 @@
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
 using System.ComponentModel.DataAnnotations.Schema;
+using Newtonsoft.Json;
 
 namespace osu.Game.Database
 {
     public interface IHasPrimaryKey
     {
+        [JsonIgnore]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
         int ID { get; set; }
     }