rust: disable borrow_as_ptr warning

This is pretty noisy, but it was not visible until now because it only shows up
if the rust-version has "&raw const".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/Cargo.toml

@@ -53,7 +53,6 @@ as_ptr_cast_mut = "deny"
 as_underscore = "deny"
 assertions_on_result_states = "deny"
 bool_to_int_with_if = "deny"
-borrow_as_ptr = "deny"
 cast_lossless = "deny"
 dbg_macro = "deny"
 debug_assert_with_mut_call = "deny"