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GH-38907: [C++] Stop installing internal bpacking_simd* headers (#38908)

### Rationale for this change

They are for internal use. We don't need to install them.

### What changes are included in this PR?

Use `_internal.h` suffix to avoid installing them.
 
### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #38907

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>