WineQualLevel: Implement GetHashCode() to avoid warning

Elwig/Models/Entities/WineQualLevel.cs

@@ -41,5 +41,9 @@ namespace Elwig.Models.Entities {
         public static bool operator !=(WineQualLevel? q1, WineQualLevel? q2) {
             return !(q1?.Equals(q2) ?? Equals(q1, q2));
         }
+
+        public override int GetHashCode() {
+            return QualId.GetHashCode();
+        }
     }
 }