Add entry in ChangeLog

Add entry in ChangeLog for compilation error fix of #1719
diff --git a/ChangeLog b/ChangeLog
index f5c0adb..38b6d9f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fix compilation error when MBEDTLS_ARC4_C is disabled and
+     MBEDTLS_CIPHER_NULL_CIPHER is enabled. Found by TrinityTonic in #1719.
+
 = mbed TLS 2.7.3 branch released 2018-04-30
 
 Security