[WCF] fix syndication extension Add() resolution by appropriate accessibility.

I wonder why, but when switched to referencesource, it turned out that
ResourceCollectionInfo calls SyndicationElementExtensionCollection.Add()
which targets a private overload, but (of course) failed and resolved to
another overload that takes object, and thus resulting in invalid code path.

With this fix the call to Add() resolves to the expected one and does not
regress with referencesource import.

mcs/class/System.ServiceModel.Web/System.ServiceModel.Syndication/SyndicationElementExtensionCollection.cs

@@ -80,7 +80,7 @@ namespace System.ServiceModel.Syndication
 			Add (new SyndicationElementExtension (xmlReader));
 		}
 
-		new void Add (SyndicationElementExtension item)
+		internal new void Add (SyndicationElementExtension item)
 		{
 			base.Add (item);
 		}