use std in main vcl too

Author: dbu

tests/Functional/Fixtures/varnish/custom_ttl.vcl

@@ -1,5 +1,7 @@
 vcl 4.0;
 
+import std;
+
 include "../../../../resources/config/varnish/fos_debug.vcl";
 include "../../../../resources/config/varnish/fos_custom_ttl.vcl";