add missing section titles in changelog

Author: svigerske

CHANGELOG

@@ -44,12 +44,18 @@ Performance improvements
 
 - Use sassy/bliss as default symmetry computation package.
 
+Interface changes
+-----------------
+
 ### Interfaces to external software
 
 - added interface to nauty/traces for symmetry computation
 - added interface to sassy, a preprocessor for symmetry computation
 - The directory src/sassy contains the source code of sassy.
 
+Build system
+------------
+
 ### Cmake
 
 - added flag option "SYM=sbliss" for using sassy/bliss as a graph automorphism package