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GH-40983: [C++] Fix unused function build error (#40984)

### Rationale for this change

Make sure local `CompressedInputArguments` is only defined when it's being used.

### What changes are included in this PR?

Wrapping definition and usage under the same `#ifdef`.

### Are these changes tested?

By building with `-Werror` and running all existing tests.
* GitHub Issue: #40983

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>