Update ChangeLog

Add ChangeLog entry describing the fix.
diff --git a/ChangeLog b/ChangeLog
index b39b953..3acb228 100644
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@@ -194,6 +194,7 @@
      replacements of standard calloc/free functions through the macros
      MBEDTLS_PLATFORM_CALLOC_MACRO and MBEDTLS_PLATFORM_FREE_MACRO.
      Reported by ole-de and ddhome2006. Fixes #882, #1642 and #1706.
+   * Reduce stack usage of hkdf tests. Fixes #2195.
 
 Changes
    * Removed support for Yotta as a build tool.