`subdir` doesn't work in included files

Using `(subdir foo <action>)` in `dune.inc` and then doing `(include dune.inc)` in `dune` results in:

```
File "dune.inc", line 3, characters 1-7:
1 | (subdir fooo
     ^^^^^^
Error: Unknown constructor subdir
```

(using dune 2.5.0)