solidcraft-smlib: the C types are imported only when SMLib is built

The previous commit left c_char and c_void unused in the build without
SMLib, which clippy -D warnings rejects.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QWhMwdqJYQKQq8RCLSL1LT

crates/smlib/src/ffi.rs

@@ -3,6 +3,7 @@
 //! root, which check their arguments and own the handles.
 #![allow(unsafe_code)]
 
+#[cfg(feature = "native")]
 use std::ffi::{c_char, c_void};
 
 /// `SMLIB_ABI_VERSION` this crate was written against.