Update CHANGELOG for PR ponylang#2645 [skip ci]

Author: ponylang-main

CHANGELOG.md

@@ -34,6 +34,7 @@ All notable changes to the Pony compiler and standard library will be documented
 
 ### Added
 
+- Add addc, subc and mulc functions to I128 and U128 ([PR #2645](https://github.com/ponylang/ponyc/pull/2645))
 - Add mechanism to register (noisy) signal handler ([PR #2631](https://github.com/ponylang/ponyc/pull/2631))
 - Added `nosupertype` annotation for subtyping exclusion (RFC 54). ([PR #2678](https://github.com/ponylang/ponyc/pull/2678))
 - `SourceLoc.type_name` method ([PR #2643](https://github.com/ponylang/ponyc/pull/2643))