[atomics.general] Add contents entry for "C compatibility" subclause

Author: tkoeppe

source/atomics.tex

@@ -20,7 +20,8 @@
 \ref{atomics.types.generic} & Class template \tcode{atomic}   &  \\
 \ref{atomics.nonmembers}  & Non-member functions &  \\
 \ref{atomics.flag}  & Flag type and operations   &  \\
-\ref{atomics.fences}  & Fences   &  \\
+\ref{atomics.fences}  & Fences   &  \\ \rowsep
+\ref{stdatomic.h.syn}  & C compatibility   & \tcode{<stdatomic.h>} \\
 \end{libsumtab}
 
 \rSec1[atomics.syn]{Header \tcode{<atomic>} synopsis}