Add sysroot CHANGELOG entry for this PR

Author: SeanTAllen

CHANGELOG.md

@@ -18,6 +18,7 @@ All notable changes to the Pony compiler and standard library will be documented
 
 - Compile-time string literal concatenation ([PR #4900](https://github.com/ponylang/ponyc/pull/4900))
 - Add `--sysroot` option ([PR #4964](https://github.com/ponylang/ponyc/pull/4964))
+- Add `--sysroot` option ([PR #4971](https://github.com/ponylang/ponyc/pull/4971))
 
 ### Changed