Fix the opaque type

The correct way seems to be to use an empty enum, which should "do the
right thing".

rust/coffi/src/main.rs

@@ -6,8 +6,8 @@ extern {
    fn cos(d: f64) -> f64;
}
#[repr(C)]
struct png_opaque;
#[allow(non_camel_case_types)]
enum png_opaque {}
#[repr(C)]
struct png_image {