foreign-library source files are not tracked in file-watch

### General summary

If cabal file contains `foreign-library` section, its source files are not tracked with `stack build --file-watch`. Stack doesn't trigger rebuild when files change.

### Steps to reproduce

```cabal
foreign-library example
build-depends:
base
hs-source-dirs: foreign
other-modules: Example
type: native-shared
```

`stack build --file-watch`

### Expected

Rebuild when foreign/Example.hs is changed.

### Actual

Nothing.

### Stack version

```
$ stack --version
Version 1.6.5 x86_64 hpack-0.20.0
```

### Method of installation

brew