Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include
diff --git a/include/mbedtls/ssl_cookie.h b/include/mbedtls/ssl_cookie.h
index 80b65bb..6a0ad4f 100644
--- a/include/mbedtls/ssl_cookie.h
+++ b/include/mbedtls/ssl_cookie.h
@@ -50,7 +50,7 @@
 /**
  * \brief          Context for the default cookie functions.
  */
-typedef struct
+typedef struct mbedtls_ssl_cookie_ctx
 {
     mbedtls_md_context_t    hmac_ctx;   /*!< context for the HMAC portion   */
 #if !defined(MBEDTLS_HAVE_TIME)