TLS 1.3: Add pre-shared-key multiple psk parser

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 381f0c4..ecc2a62 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -680,6 +680,7 @@
     unsigned char *psk;                 /*!<  PSK from the callback         */
     size_t psk_len;                     /*!<  Length of PSK from callback   */
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
+    uint16_t    selected_identity;
 #endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */
 
 #if defined(MBEDTLS_SSL_ECP_RESTARTABLE_ENABLED)