Change JxlBoxType to use system char type (#77)

On Linux ARM64 machines, compilation fails with the following error:
```
error[E0308]: mismatched types
  --> jpegxl-rs/src/encode/metadata.rs:22:29
   |
22 |         JxlBoxType(unsafe { std::mem::transmute::<[u8; 4], [i8; 4]>(t) })
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `[u8; 4]`, found `[i8; 4]`
   |
   = note: expected array `[u8; 4]`
              found array `[i8; 4]`
```

The `JxlBoxType` is a new-type around `[std::ffi::c_char; 4]`. On many
platforms, `std::ffi::c_char` is an `i8`. However, as the
[docs](https://doc.rust-lang.org/stable/std/ffi/type.c_char.htm) state,
some platforms will define `c_char` as a `u8`. Indeed, when looking at
the
[source](https://doc.rust-lang.org/1.80.1/src/core/ffi/mod.rs.html#83),
we can see that linux/aarch64 (among other platforms) defines a `c_char`
as `u8`.

The proposed solution simply changes the `transmute` call in
`src/encode/metadata.rs` to target `c_char` instead of `i8`, so
compilation succeeds regardless of whether `c_char` is defined as `u8`
or `i8`.

jpegxl-rs/src/encode/metadata.rs

@@ -19,6 +19,6 @@ pub enum Metadata<'d> {
 impl Metadata<'_> {
     #[must_use]
     pub(crate) fn box_type(t: [u8; 4]) -> JxlBoxType {
-        JxlBoxType(unsafe { std::mem::transmute::<[u8; 4], [i8; 4]>(t) })
+        JxlBoxType(unsafe { std::mem::transmute::<[u8; 4], [std::ffi::c_char; 4]>(t) })
     }
 }