Fix wrong pointer type of static _dispatch_main_q binding

While looking at a copy of this code (in another project that uses
`objc2`, which does not yet define `dispatch` integration) this strange
cast immediately stands out.  `_dispatch_main_q` is defined here as a
`dispatch_queue_t`, the `queue` argument in `dispatch_data_create()`
is also defined as a `dispatch_queue_t`, yet there is a "borrow" (get
*pointer to*) operation before passing it into the function.

Instead, this `static` field is supposed to be defined as the
`Object` itself, so that any user could take a _pointer to it_
if they wish or need it so.  The same is seen in [the `dispatch`
crate]: `_dispatch_main_q` is defined as a `dispatch_object_s`, and
`dispatch_queue_t` is a _typedef_ to a `*mut dispatch_object_s` to match
the above convention.

Note that this is not a bugfix, but merely a readability improvement.
Use of `mut` here is debatable.

[the `dispatch` crate]: https://github.com/SSheldon/rust-dispatch/blob/f540a2d8ccaebf0e87f5805033b9e287e8d01ba5/src/ffi.rs#L33

src/device.rs

@@ -1475,16 +1475,22 @@ type dispatch_block_t = *const Block<(), ()>;
 const DISPATCH_DATA_DESTRUCTOR_DEFAULT: dispatch_block_t = ptr::null();
 
 #[cfg_attr(
-    all(feature = "link", any(target_os = "macos", target_os = "ios", target_os = "visionos")),
+    all(
+        feature = "link",
+        any(target_os = "macos", target_os = "ios", target_os = "visionos")
+    ),
     link(name = "System", kind = "dylib")
 )]
 #[cfg_attr(
-    all(feature = "link", not(any(target_os = "macos", target_os = "ios", target_os = "visionos"))),
+    all(
+        feature = "link",
+        not(any(target_os = "macos", target_os = "ios", target_os = "visionos"))
+    ),
     link(name = "dispatch", kind = "dylib")
 )]
 #[allow(improper_ctypes)]
 extern "C" {
-    static _dispatch_main_q: dispatch_queue_t;
+    static mut _dispatch_main_q: Object;
 
     fn dispatch_data_create(
         buffer: *const std::ffi::c_void,
@@ -1719,7 +1725,7 @@ impl DeviceRef {
             let data = dispatch_data_create(
                 library_data.as_ptr() as *const std::ffi::c_void,
                 library_data.len() as crate::c_size_t,
-                &_dispatch_main_q as *const _ as dispatch_queue_t,
+                &mut _dispatch_main_q,
                 DISPATCH_DATA_DESTRUCTOR_DEFAULT,
             );