capnpc-capnp: keep list of extended interfaces.

```
interface BasicCap @0xf329462caa09f38f {
  add @0 (a :Int64, b :Int64) -> (result :Int64);
}
interface OtherCap @0x9000899726987e7f {
  sqroot @0 (a :Int64, b :Int64) -> (root1 :Int64, root2 :Int64);
}
interface ThirdCap @0xfb84e23a9ca71e0e extends(BasicCap, OtherCap) {
  square @0 (a :Int64) -> (sq :Int64);
}
```

Without this patch, the `extends()` info for `ThirdCap` is missing in the output from `capnpc-capnp`.