struct in a struct with __attribute__((packed))

Support for either of these cases:

```
struct foo_t {
    struct __attribute__((packed)) {
        int a;
        char b;
    };
};
```

```
struct foo_t {
    struct {
        int a;
        char b;
    } __attribute__((packed));
};
```

I can work on an patch, but just posting an issue so it's documented.