Add missing setter for conf->new_session_tickets_count (#313)

This option determines the number of new session tickets generated for
each session, and is only present when `MBEDTLS_SSL_PROTO_TLS1_3` is set.

We should add a setter for this option when the `tls13` feature is
enabled.

mbedtls/src/ssl/config.rs

@@ -371,6 +371,11 @@ impl Config {
         set_session_tickets(u: UseSessionTickets) = ssl_conf_session_tickets
     );
 
+    #[cfg(feature = "tls13")]
+    setter!(
+        set_new_session_tickets_count(cnt: u16) = ssl_conf_new_session_tickets
+    );
+
     setter!(set_renegotiation(u: Renegotiation) = ssl_conf_renegotiation);
 
     setter!(