GH-50139: [Dev][Gandiva] Add Gandiva code owners (#50144)

### Rationale for this change

We have Gandiva PRs that wait for review.

If we have Gandiva code owners, they can notice them easily.

### What changes are included in this PR?

Add Gandiva code owners.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* GitHub Issue: #50139

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: kou

.github/CODEOWNERS

@@ -59,6 +59,7 @@ compose.yaml @assignUser @jonkeane @kou @raulcd
 /cpp/src/arrow/adapters/orc @wgtmac
 /cpp/src/arrow/engine @westonpace
 /cpp/src/arrow/flight/ @lidavidm
+/cpp/src/gandiva @dmitry-chirkov-dremio @lriggs @akravchukdremio @xxlaykxx
 /cpp/src/parquet @wgtmac @pitrou
 
 ## MATLAB