LWG3987 Including <flat_foo> doesn't provide std::begin/end

Author: burblebee

source/iterators.tex

@@ -7215,6 +7215,8 @@
 headers are included:
 \libheaderref{array},
 \libheaderref{deque},
+\libheaderrefx{flat_map}{flat.map.syn},
+\libheaderrefx{flat_set}{flat.set.syn},
 \libheaderrefx{forward_list}{forward.list.syn},
 \libheaderref{list},
 \libheaderrefx{map}{associative.map.syn},