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/hw/timer/hpet/src/hpet.rs

@@ -219,7 +219,7 @@ impl HPETTimer {
             // SAFETY: the HPETTimer will only be used after the timer
             // is initialized below.
             qemu_timer: unsafe { Timer::new() },
-            state: NonNull::new(state as *const _ as *mut _).unwrap(),
+            state: NonNull::new((state as *const HPETState).cast_mut()).unwrap(),
             config: 0,
             cmp: 0,
             fsb: 0,