rename `ticket_creation` to `ticket_creation_time`

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_ticket.c b/library/ssl_ticket.c
index 5fef4eb..0277bfa 100644
--- a/library/ssl_ticket.c
+++ b/library/ssl_ticket.c
@@ -498,7 +498,7 @@
 #if defined(MBEDTLS_SSL_PROTO_TLS1_3)
     if (session->tls_version == MBEDTLS_SSL_VERSION_TLS1_3) {
         /* Check for expiration */
-        mbedtls_ms_time_t ticket_age = mbedtls_ms_time() - session->ticket_creation;
+        mbedtls_ms_time_t ticket_age = mbedtls_ms_time() - session->ticket_creation_time;
         mbedtls_ms_time_t ticket_lifetime = ctx->ticket_lifetime * 1000;
 
         if (ticket_age < 0 || ticket_age > ticket_lifetime) {