rust: enable clippy::ptr_cast_constness

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/chardev.rs

@@ -160,7 +160,7 @@ impl CharBackend {
                 receive_cb,
                 event_cb,
                 None,
-                (owner as *const T as *mut T).cast::<c_void>(),
+                (owner as *const T).cast_mut().cast::<c_void>(),
                 core::ptr::null_mut(),
                 true,
             );