Update Changelog for API/ABI fixes to revert interface

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 96f4b31..e7b596f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,12 @@
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+API Changes
+   * Reverted API/ABI breaking changes introduced in mbed TLS 2.5.1, to make the
+     API consistent with mbed TLS 2.5.0. Specifically removed the inline
+     qualifier from the functions mbedtls_aes_decrypt, mbedtls_aes_encrypt,
+     mbedtls_ssl_ciphersuite_uses_ec and mbedtls_ssl_ciphersuite_uses_psk.
+
 Bugfix
    * Add a check if iv_len is zero, and return an error if it is zero. reported
      by roberto. #716