Fix build error

Ix.NET/Source/System.Interactive.Providers/System/Linq/QueryableEx.Generated.cs

@@ -2152,7 +2152,11 @@ namespace System.Linq
         [EditorBrowsable(EditorBrowsableState.Never)]
         public static IEnumerable<TSource> TakeLast<TSource>(IEnumerable<TSource> source, int count)
         {
+#if REFERENCE_ASSEMBLY
+            return null;
+#else
             return EnumerableEx.TakeLast(source, count);
+#endif
         }
 #pragma warning restore 1591
 
@@ -2186,7 +2190,11 @@ namespace System.Linq
         [EditorBrowsable(EditorBrowsableState.Never)]
         public static IEnumerable<TSource> SkipLast<TSource>(IEnumerable<TSource> source, int count)
         {
+#if REFERENCE_ASSEMBLY
+            return null;
+#else
             return EnumerableEx.SkipLast(source, count);
+#endif
         }
 #pragma warning restore 1591