Of course you should put a letter to the editor in your CV. However, it should be under the section: "Editorials and Reviews," or something like that, so that it is clear that it is not peer reviewed. By the same token, your list of peer-reviewed publications should be called as such and should not include any editorials, review articles, invited commentaries, or book chapters, unless these have been peer reviewed. The real distinction is whether a publication has been peer-reviewed.