TLS1.3: add writing encrypted extensions

Signed-off-by: Xiaofei Bai <xiaofei.bai@arm.com>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 141c40a..abf33a6 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -809,6 +809,14 @@
             size_t digest_len;
         } finished_in;
 
+#if defined(MBEDTLS_SSL_SRV_C)
+        /* Server, outgoing ClientKeyExchange */
+        struct
+        {
+            uint8_t preparation_done;
+        } encrypted_extensions_out;
+#endif /* MBEDTLS_SSL_SRV_C */
+
     } state_local;
 
     /* End of state-local variables. */