rust: pin-init: examples: error: use Error in fn main()

When running this example with no cargo features enabled, the compiler
warns on 1.89:

  error: struct `Error` is never constructed
    --> examples/error.rs:11:12
     |
  11 | pub struct Error;
     |            ^^^^^
     |
     = note: `-D dead-code` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(dead_code)]`

Thus use the error in the main function to avoid this warning.

Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/pin-init/examples/error.rs

@@ -24,4 +24,6 @@ fn from(_: AllocError) -> Self {
 }
 
 #[allow(dead_code)]
-fn main() {}
+fn main() {
+    let _ = Error;
+}