rust: fix empty struct bindgen issue

evmc_host_context cannot be instantiated because original statement will generate private field in struct.

bindings.rs
```rust
pub struct evmc_host_context {
    _unused: [u8; 0],
}
```

include/evmc/evmc.h

@@ -161,7 +161,7 @@ struct evmc_tx_context
  * The opaque data type representing the Host execution context.
  * @see evmc_execute_fn().
  */
-struct evmc_host_context;
+struct evmc_host_context {};
 
 /**
  * Get transaction context callback function.