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_srv.c b/library/ssl_srv.c
index c8f0e7f..61c3e6a 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -19,9 +19,7 @@
 
 #include "common.h"
 
-#if defined(MBEDTLS_SSL_SRV_C)
-
-#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
+#if defined(MBEDTLS_SSL_SRV_C) && defined(MBEDTLS_SSL_PROTO_TLS1_2)
 
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
@@ -4408,6 +4406,4 @@
     conf->respect_cli_pref = order;
 }
 
-#endif /* MBEDTLS_SSL_PROTO_TLS1_2 */
-
-#endif /* MBEDTLS_SSL_SRV_C */
+#endif /* MBEDTLS_SSL_SRV_C && MBEDTLS_SSL_PROTO_TLS1_2 */