Fix struct signature

Repro: 

```
pub struct A {
    pub b: felt252,
    c: felt252,
}
```

will have a signature generated as:
```
pub struct A {
    pub b: felt252,
    c: felt252,
}
``` 
instead of 

```
pub struct A {
    pub b: felt252,
}
```

whereas the members have the visibility checked properly. 
