add substratevm/CHANGELOG.md entry for new -Werror flag

Author: wirthi

substratevm/CHANGELOG.md

@@ -3,7 +3,8 @@
 This changelog summarizes major changes to GraalVM Native Image.
 
 ## GraalVM for JDK 26 (Internal Version 26.0.0)
-* (GR-69280): Allow use of the `graal.` prefix for options without issuing a warning.
+* (GR-43070) Add a new API flag `-Werror` to treat warnings as errors.
+* (GR-69280) Allow use of the `graal.` prefix for options without issuing a warning.
 
 ## GraalVM 25
 * (GR-52276) (GR-61959) Add support for Arena.ofShared().