GH-49081: [C++][Parquet][FOLLOWUP] Correct variant's extension name

[HuaHuaY]

Rationale for this change

Previous PR moved one file, modified the CMakeLists but didn't modify the meson file. Fix meson build failure.

What changes are included in this PR?

Modified the meson configuration files.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Parquet] Wrong variant extension name #49081

[HuaHuaY]

@WillAyd Current CI does not check whether code can be compiled by meson successfully. Do we need to add a CI to check this?

[Copilot]

Pull request overview

Updates Meson build definitions in the C++ Parquet/Arrow codebase to reflect a previous file move/rename related to the Parquet Variant extension, fixing Meson build breakage.

Changes:

• Remove the outdated arrow/variant_internal.cc entry from Parquet’s Meson source list.
• Add extension/parquet_variant.cc to Arrow’s Meson source list so the moved implementation is compiled into Arrow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cpp/src/parquet/meson.build	Drops a source file entry that no longer exists/was moved, preventing Meson build errors.
cpp/src/arrow/meson.build	Adds the new Parquet Variant extension implementation source to Arrow’s build sources.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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[wgtmac]

Thanks for fixing this!

[WillAyd]

@WillAyd Current CI does not check whether code can be compiled by meson successfully. Do we need to add a CI to check this?

For Meson to run the C++ Extra tag has to be used