RFR: 8323629: Shenandoah: Fix missing include and declaration

[William Kemper]

There are some small changes for style guide conformance here.

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Commit messages:
 - Fix missing headers and declarations and conform to style guide

Changes: https://git.openjdk.org/jdk/pull/17384/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17384&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323629
  Stats: 18 lines in 3 files changed: 13 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/17384.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17384/head:pull/17384

PR: https://git.openjdk.org/jdk/pull/17384