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GH-37107: [C++] Suppress an unused variable warning with GCC 7 (#37240)

### Rationale for this change

If there is a warning, debug build is failed by default.

### What changes are included in this PR?

Suppress the unused warning explicitly. 

### Are these changes tested?

No.

### Are there any user-facing changes?

Yes.
* Closes: #37107

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>