Add Changelog entry for clang test-ref-configs.pl fix
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+ * Fix to allow test-ref-config.pl to build with Clang. Fixes #1628.
+
= mbed TLS 2.16.0 branch released 2018-12-21
Features
