C-variadic has been used on a non-foreign function.

Erroneous code example:

```compile_fail,E0743
fn foo2(x: u8, ...) {} // error!
```

Only foreign functions can use C-variadic (`...`). It is used to give an
undefined number of parameters to a given function (like `printf` in C). The
equivalent in Rust would be to use macros directly.
