Constify parameter of ssl_tls13_session_load

Signed-off-by: Norbert Fabritius <norbert.fabritius@esrlabs.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index c5e0649..931fdaf 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4049,7 +4049,7 @@
 }
 
 static int ssl_tls13_session_load(const mbedtls_ssl_session *session,
-                                  unsigned char *buf,
+                                  const unsigned char *buf,
                                   size_t buf_len)
 {
     ((void) session);