Add ChangeLog entry
diff --git a/ChangeLog b/ChangeLog
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@@ -17,6 +17,7 @@
    * Fix out-of-memory problem when parsing 4096-bit PKCS8-encrypted RSA keys.
      Found independently by Florian in the mbed TLS forum and by Mishamax.
      #878, #1019.
+   * Fix build problems on Windows. Contributed by Nicholas Wilson.
 
 = mbed TLS 1.3.21 branch released 2017-08-10