add debug messages

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 418fc13..62f22fd 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -940,6 +940,11 @@
             mbedtls_ssl_tls13_check_sig_alg_cert_key_match(
                                                         *sig_alg, own_key ) )
         {
+            MBEDTLS_SSL_DEBUG_MSG(
+                    3, ( "select_sig_alg_for_certificate_verify:"
+                         "selected signature algorithm %s [%04x]",
+                         mbedtls_ssl_sig_alg_to_str( *sig_alg ),
+                         *sig_alg ) );
             *algorithm = *sig_alg;
             return( 0 );
         }