add named group debug helper

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_debug_helpers.h b/library/ssl_debug_helpers.h
index 29b64dc..9f1df73 100644
--- a/library/ssl_debug_helpers.h
+++ b/library/ssl_debug_helpers.h
@@ -41,6 +41,8 @@
 
 const char *mbedtls_ssl_sig_alg_to_str( uint16_t in );
 
+const char *mbedtls_ssl_named_group_to_str( uint16_t in );
+
 #endif /* MBEDTLS_DEBUG_C */
 
 #endif /* SSL_DEBUG_HELPERS_H */