[diff.cpp20.library] Add missing new headers

Fixes NB FR 008 (C++23 CD).

Author: tkoeppe

source/compatibility.tex

@@ -128,9 +128,13 @@
 \effect
 The following \Cpp{} headers are new:
 \libheaderref{expected},
-\libheaderref{stdatomic.h},
-\libheaderref{spanstream}, and
-\libheaderref{stacktrace}.
+\libheaderrefx{flat_map}{flat.map.syn},
+\libheaderrefx{flat_set}{flat.set.syn},
+\libheaderref{generator},
+\libheaderref{print},
+\libheaderref{spanstream},
+\libheaderref{stacktrace}, and
+\libheaderref{stdatomic.h}.
 Valid \CppXX{} code that \tcode{\#include}{s} headers with these names may be
 invalid in this revision of \Cpp{}.