RFR: 8329878: Reduce public interface of CardTableBarrierSet

[Albert Mingkun Yang]

Trivial moving typedef from public to protected.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18679/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18679&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329878
  Stats: 4 lines in 1 file changed: 1 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18679.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18679/head:pull/18679

PR: https://git.openjdk.org/jdk/pull/18679