Use c_uint instead of uint32_t

It's closer to `unsigned int`, which it is declared as in the libpng
headers.

rust/coffi/src/main.rs

@@ -9,13 +9,13 @@ extern {
#[repr(C)]
struct png_image {
    opaque: *mut libc::c_void,
    version: libc::uint32_t,
    width: libc::uint32_t,
    height: libc::uint32_t,
    format: libc::uint32_t,
    flags: libc::uint32_t,
    colormap_entries: libc::uint32_t,
    warning_or_error:  libc::uint32_t,
    version: libc::c_uint,
    width: libc::c_uint,
    height: libc::c_uint,
    format: libc::c_uint,
    flags: libc::c_uint,
    colormap_entries: libc::c_uint,
    warning_or_error:  libc::c_uint,
    message: [libc::c_char; 64],
}