Update ChangeLog

Add an entry in ChangeLog describing the fix.
diff --git a/ChangeLog b/ChangeLog
index 8f0e8c1..00298a2 100644
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+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.xx.x branch released xxxx-xx-xx
+
+Bugfix
+   * Run the AD too long test only if MBEDTLS_CCM_ALT is not defined.
+     Raised as a comment in #1996.
+
 = mbed TLS 2.14.0 branch released 2018-11-19
 
 Security