tls13_only: improve guards of files.

To improve readability of the preprocess guards.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index f556c0f..d599ef3 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -21,9 +21,7 @@
 
 #include "common.h"
 
-#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
-
-#if defined(MBEDTLS_SSL_CLI_C)
+#if defined(MBEDTLS_SSL_CLI_C) &&defined(MBEDTLS_SSL_PROTO_TLS1_3)
 
 #include <string.h>
 
@@ -2059,6 +2057,6 @@
     return( ret );
 }
 
-#endif /* MBEDTLS_SSL_CLI_C */
+#endif /* MBEDTLS_SSL_CLI_C && MBEDTLS_SSL_PROTO_TLS1_3 */
 
-#endif /* MBEDTLS_SSL_PROTO_TLS1_3 */
+