[diff.cpp14.library] Add Annex C entry for new headers in C++17

Fixes #1700.

Author: timsong-cpp

source/compatibility.tex

@@ -1690,6 +1690,22 @@
 
 \rSec2[diff.cpp14.library]{\ref{library}: library introduction}
 
+\ref{headers}
+\change New headers.
+\rationale New functionality.
+\effect
+The following \Cpp headers are new:
+\tcode{<any>},
+\tcode{<execution>},
+\tcode{<filesystem>},
+\tcode{<memory_resource>},
+\tcode{<optional>},
+\tcode{<string_view>},
+and
+\tcode{<variant>}.
+Valid \CppXIV code that \tcode{\#include}{s} headers with these names may be
+invalid in this International Standard.
+
 \ref{namespace.future}
 \change New reserved namespaces.
 \rationale Reserve namespaces for future revisions of the standard library