ParseXS: fix under older perls

One line had: 'exists $param->{type}' as part of a condition. This
worked fine when $param was a normal hash ref (created under 'use
fields'), but the combination of:

- a recent commit which copies those hashes to $sig->{old_params}
  then resets some of the fields (to allow for multiple CASEs);

- older perls (in particular 5.8.9) which used pseudo-hashes to
  implement fields,

was causing the field to spring into existence, even though it wasn't defined.
So change the test from 'exists' to 'defined'.

Author: iabyn

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS.pm

@@ -2008,7 +2008,7 @@ sub INPUT_handler {
       # synthetic params like THIS, which are assigned a provisional type
       # which can be overridden.
       if (   $param->{in_input}
-          or (!$param->{is_synthetic} and exists $param->{type})
+          or (!$param->{is_synthetic} and defined $param->{type})
       ) {
           $self->blurt(
             "Error: duplicate definition of parameter '$var_name' ignored");