privacy: Add publically-reexported foreign item to exported item set

Close #15740
Thanks. I know @alexcrichton also preferred to limit himself to 80 characters when appropriate, but we have recently officially removed the 80/100 split and settled on 99 (although that 99 is not set in stone yet). I wouldn't have said anything except that the line continuation here ended up indented only 1 level, which matches the indentation of the body. If it didn't fit on one line, I would have instead suggested indenting it a second level.