Add ChangeLog entry.

Fixes #1353
diff --git a/ChangeLog b/ChangeLog
index 68fb6f5..b88048d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -35,6 +35,8 @@
      In the context of SSL, this resulted in handshake failure. Reported by
      daniel in the Mbed TLS forum. #1351
    * Fix Windows x64 builds with the included mbedTLS.sln file. #1347
+   * Fix C89 incompatibility in benchmark.c. Contributed by Brendan Shanks.
+     #1353
 
 Changes
    * Fix tag lengths and value ranges in the documentation of CCM encryption.