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

[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.