Add md/shaXXX_clone() API

Will be used in the SSL/TLS modules
diff --git a/library/md5.c b/library/md5.c
index 599debc..7c846be 100644
--- a/library/md5.c
+++ b/library/md5.c
@@ -89,6 +89,12 @@
     mbedtls_zeroize( ctx, sizeof( mbedtls_md5_context ) );
 }
 
+void mbedtls_md5_clone( mbedtls_md5_context *dst,
+                        const mbedtls_md5_context *src )
+{
+    *dst = *src;
+}
+
 /*
  * MD5 context setup
  */