Restore changelog entry mistakenly removed in emscripten-core#23017. NFC (emscripten-core#23071)

Author: kleisauke

ChangeLog.md

@@ -24,6 +24,7 @@ See docs/process.md for more on how version tagging works.
 3.1.73 - 11/28/24
 -----------------
 - libunwind was updated to LLVM 19.1.4. (#22394)
+- mimalloc was updated to 2.1.7. (#21548)
 
 3.1.72 - 11/19/24
 -----------------