Do not add a new field in the SSL config

We cannot add a new field in SSL config in
an LTS. Use `session_tickets` field instead.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index eac6a3a..9b2da5a 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -364,7 +364,8 @@
 
     *olen = 0;
 
-    if (ssl->conf->session_tickets == MBEDTLS_SSL_SESSION_TICKETS_DISABLED) {
+    if (mbedtls_ssl_conf_get_session_tickets(ssl->conf) ==
+        MBEDTLS_SSL_SESSION_TICKETS_DISABLED) {
         return 0;
     }
 
@@ -787,7 +788,8 @@
                                         const unsigned char *buf,
                                         size_t len)
 {
-    if (ssl->conf->session_tickets == MBEDTLS_SSL_SESSION_TICKETS_DISABLED ||
+    if ((mbedtls_ssl_conf_get_session_tickets(ssl->conf) ==
+         MBEDTLS_SSL_SESSION_TICKETS_DISABLED) ||
         len != 0) {
         MBEDTLS_SSL_DEBUG_MSG(1,
                               ("non-matching session ticket extension"));