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GH-45354: [GLib] Fix garrow_record_batch_validate() definied location (#45355)



### Rationale for this change

#45353 implemented `garrow_record_batch_validate()` method. But it was the incorrect location.
We need to move the code location to use it as a C API.

### What changes are included in this PR?

Move the `garrow_record_batch_validate()` location between `G_BEGIN_DECLS` and `G_END_DECLS`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.

* GitHub Issue: #45354

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