Fix warning with MD/SHA ALT implementation

backport of 8b2641d

see #239

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 2c369ce..b22fdde 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,8 @@
      result trying to unlock an unlocked mutex on invalid input (found by
      Fredrik Axelsson) (#257)
    * Fix -Wshadow warnings (found by hnrkp) (#240)
+   * Fix unused function warning when using MBEDTLS_MDx_ALT or
+     MBEDTLS_SHAxxx_ALT (found by Henrik) (#239)
 
 = mbed TLS 1.3.12 released 2015-08-11