rust: vmstate: use "cast()" instead of "as"

Reported by clippy, fix it.

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

rust/migration/src/vmstate.rs

@@ -144,7 +144,7 @@ macro_rules! vmstate_of {
         $crate::bindings::VMStateField {
             name: ::core::concat!(::core::stringify!($field_name), "\0")
                 .as_bytes()
-                .as_ptr() as *const ::std::os::raw::c_char,
+                .as_ptr().cast::<::std::os::raw::c_char>(),
             offset: ::std::mem::offset_of!($struct_name, $field_name),
             $(num_offset: ::std::mem::offset_of!($struct_name, $num),)?
             $(field_exists: $crate::vmstate_exist_fn!($struct_name, $test_fn),)?