[MINOR][VL] Flatten backends-velox pom

[wecharyu]

What changes are proposed in this pull request?

Enable flatten-maven-plugin for backends-velox.

This is similar to #11958, when running following test command in Spark 4.x, maven resolved arrow version 15, while Spark 4.x requires arrow 18, then the test failed:

 java.lang.NoClassDefFoundError: org/apache/arrow/vector/types/pojo/ArrowType$Utf8View
        at org.apache.arrow.c.SchemaExporter.export(SchemaExporter.java:70)
        at org.apache.arrow.c.Data.exportField(Data.java:55)
        at org.apache.arrow.c.Data.exportSchema(Data.java:72)
        at org.apache.gluten.utils.ArrowAbiUtil$.exportSchema(ArrowAbiUtil.scala:134)
        at org.apache.gluten.execution.RowToVeloxColumnarExec$.toColumnarBatchIterator(RowToVeloxColumnarExec.scala:140)

How was this patch tested?

Pass test:

build/mvn test -Dtest=none -Pspark-4.1 -Pscala-2.13 -Pjava-17 -Pbackends-velox -Pspark-ut -DargLine="-Dspark.test.home=/opt/shims/spark41/spark_home/ -Dspark.testing=true" -pl gluten-ut/spark41 -Dsuites="org.apache.spark.sql.execution.datasources.parquet.GlutenParquetIOSuite"

Was this patch authored or co-authored using generative AI tooling?

No.

[philo-he]

LGTM. Thanks.