Fix compilation failure with clang 12 and debug=yes
Fix compilation failure with clang 12 and debug=yes

The pointer type alias was missing from FlattenedView::Iterator causing
std::iterator_traits to be an empty class.

jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune,jkonecny12/kakoune