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GH-40500: [C++] Ensure using bundled FlatBuffers (#40519)

### Rationale for this change

We'rebundling FlatBuffers and generated files by FlatBuffers. If we use system FlatBuffers that is different version of bundled one, we got a build error. 

### What changes are included in this PR?

Use `arrow::flatbuffers` as the first link library to prioritize bundled FlatBuffers than system FlatBuffers.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #40500

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