Integrated: 8338420: GenShen: Forward declare card table for Shenandoah barrier set

[William Kemper]

Trivial include conflict.

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Commit messages:
 - Fix include
 - Backport 5c94dd648423351837fd6ebfb4e3fec2f6d79f68

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/83/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=83&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338420
  Stats: 12 lines in 6 files changed: 8 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/83.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/83/head:pull/83

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/83