undefined method 'value=' for struct member

``` ruby
class Foo < CFunc::Struct
  define CFunc::Int,:int
end
class Bar < CFunc::Struct
  define Foo,:foo
end
b=Bar.new

#will raise here
b[:foo] = Foo.new
#
b[:foo][:int]=1
p b[:foo][:int] #=> 1

```
