Add entry describing the bug fix

Add entry describing the bug fix in `mbedtls_ecdsa_genkey()`.
diff --git a/ChangeLog b/ChangeLog
index 8eab14b..c8cba4a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -15,6 +15,7 @@
    * Add explicit integer to enumeration type casts to example program
      programs/pkey/gen_key which previously led to compilation failure
      on some toolchains. Reported by phoenixmcallister. Fixes #2170.
+   * Fix returning the value 1 when mbedtls_ecdsa_genkey failed.
 
 = mbed TLS 2.7.8 branch released 2018-11-30