RFR: Alphabetize includes

[William Kemper]

Follow up to feedback on https://github.com/openjdk/shenandoah/pull/269

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Commit messages:
 - Alphabetize includes

Changes: https://git.openjdk.org/shenandoah/pull/271/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=271&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/271.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/271/head:pull/271

PR: https://git.openjdk.org/shenandoah/pull/271