[diff.cpp17.library] Add Annex C entry for new headers <compare> and

<syncstream> added since C++17.

Author: zygoloid

source/compatibility.tex

@@ -1896,6 +1896,19 @@
 }
 \end{codeblock}
 
+\rSec2[diff.cpp17.library]{\ref{library}: library introduction}
+
+\ref{headers}
+\change New headers.
+\rationale New functionality.
+\effect
+The following \Cpp{} headers are new:
+\tcode{<compare>}
+and
+\tcode{<syncstream>}.
+Valid \CppXVII{} code that \tcode{\#include}{s} headers with these names may be
+invalid in this International Standard.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \rSec1[diff.library]{C standard library}
 \indextext{library!C standard}%