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/timer.rs

@@ -81,7 +81,7 @@ impl Timer {
                 scale as c_int,
                 attributes as c_int,
                 Some(timer_cb),
-                (opaque as *const T).cast::<c_void>() as *mut c_void,
+                (opaque as *const T).cast::<c_void>().cast_mut(),
             )
         }
     }