Prefer g_get_real_time() over time(NULL)

Especially when casting the result to guint64, since this might not be
always correct.

See also issue #1402.

gio/tests/gdbus-example-unix-fd-client.c

@@ -79,7 +79,7 @@ on_name_appeared (GDBusConnection *connection,
       gssize len;
       gchar *str;
 
-      now = time (NULL);
+      now = g_get_real_time ();
       strftime (now_buf,
                 sizeof now_buf,
                 "%Y-%m-%d %H:%M:%S",