Add entry for PR #1646, for IAR Compiler Warnings fix

PR #1655 is a change to the behaviour of the CMake files therefore should be
recorded in the Changelog.
diff --git a/ChangeLog b/ChangeLog
index db4669d..2d33641 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -8,6 +8,7 @@
 
 Changes
    * Support TLS testing in out-of-source builds using cmake. Fixes #1193.
+   * Changed CMake defaults for IAR to treat all compiler warnings as errors.
 
 = mbed TLS 2.7.3 branch released 2018-04-30