Use libc::c_void for the opaque struct

We can use this because we don't ever want to access it.

rust/coffi/src/main.rs

@@ -6,12 +6,9 @@ extern {
    fn cos(d: f64) -> f64;
}
#[allow(non_camel_case_types)]
enum png_opaque {}
#[repr(C)]
struct png_image {
    opaque: *mut png_opaque,
    opaque: *mut libc::c_void,
    version: libc::uint32_t,
    width: libc::uint32_t,
    height: libc::uint32_t,