Ensure CAPNP_LITE is always defined to 0 when not used

With `-Wundef`, Clang (and probably GCC, too) raises a warning on the use of `#if !CAPNP_LITE` in the generated sources for schemas:
```
warning: 'CAPNP_LITE' is not defined, evaluates to 0 [-Wundef]
#if !CAPNP_LITE
     ^
```

Rather than switching to `#if !defined(CAPNP_LITE)` or `#if !defined(CAPNP_LITE) || !CAPNP_LITE`, this seems like the tidiest way to fix it.