Nested includes show up as unused in FunC

Example:
```func
;; main.fc

#include "test1.fc";

() main() {
~dump(foo());
}
```

```func
;; test1.fc

#include "test2.fc";
```

```func
;; test2.fc

int foo() {
return 1;
}
```

Include in main.fc will show up as unused.