Add md/shaXXX_clone() API

Will be used in the SSL/TLS modules
diff --git a/library/ripemd160.c b/library/ripemd160.c
index d8a451d..ecaeea0 100644
--- a/library/ripemd160.c
+++ b/library/ripemd160.c
@@ -88,6 +88,12 @@
     mbedtls_zeroize( ctx, sizeof( mbedtls_ripemd160_context ) );
 }
 
+void mbedtls_ripemd160_clone( mbedtls_ripemd160_context *dst,
+                        const mbedtls_ripemd160_context *src )
+{
+    *dst = *src;
+}
+
 /*
  * RIPEMD-160 context setup
  */