chore: Fix error by cfg-guarding the declaration of filename and not just its use

examples/write-large-file.rs

@@ -9,9 +9,9 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         return Err(format!("Usage: {} <filename>", args[0]).into());
     }
     
-    let filename = &*args[1];
     #[cfg(feature = "_deflate-any")]
     {
+        let filename = &*args[1];
         doit(filename)?;
     }
     Ok(())