.gch not cleaned up properly

#### Summary:

When running `make clean-all` I'm getting 

```
rm: cannot remove 'stan/src/stan/model/model_header.hpp.gch': Is a directory
make: *** [makefile:286: clean] Error 1
```

This should be getting cleanup up properly


#### Reproducible Steps:

```
make build
make clean-all
```

#### Current Version:
v2.35.0
