ssl_tls13: fix guard for FFDH function

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index a063084..20cecdb 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -1521,6 +1521,7 @@
     return 0;
 }
 
+#if defined(PSA_WANT_ALG_FFDH)
 static psa_status_t  mbedtls_ssl_get_psa_ffdh_info_from_tls_id(
     uint16_t tls_id, size_t *bits, psa_key_type_t *key_type)
 {
@@ -1549,6 +1550,7 @@
             return PSA_ERROR_NOT_SUPPORTED;
     }
 }
+#endif /* PSA_WANT_ALG_FFDH */
 
 int mbedtls_ssl_tls13_generate_and_write_xxdh_key_exchange(
     mbedtls_ssl_context *ssl,