explicit prototype for contains_deferred

Hi,

perl complains that contains_deferred is called too early, probably because it's recursive. Adding an explicit prototype fixes that.

```
--- a/src/ferm
+++ b/src/ferm
@@ -1730,6 +1730,8 @@ sub eval_bool($) {
     }
 }
 
+sub contains_deferred(@);
+
 sub contains_deferred(@) {
     foreach my $value (@_) {
         if (ref $value) {
```

Greetings
Marc
