Add ChangeLog entry for PR #1555

Adds ChangeLog entry for PR #1555, 'Only redefine _WIN32_WINNT macro when <
0x0501'
diff --git a/ChangeLog b/ChangeLog
index 8061792..cc1e51c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -12,6 +12,8 @@
 Bugfix
    * Fix the cert_write example to handle certificates signed with elliptic
      curves as well as RSA. Fixes #777 found by dbedev.
+   * Fix for redefinition of _WIN32_WINNT to avoid overriding a definition
+     used by user applications. Found and fixed by Fabio Alessandrelli.
 
 Changes
    * Changed CMake defaults for IAR to treat all compiler warnings as errors.