RFR: 8339149: jfr_flush_event_writer - return value type mismatch

[Markus Grönlund]

Very small adjustment to the function signature.

Testing: jdk_jfr

Thanks
Markus

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Commit messages:
 - 8339149

Changes: https://git.openjdk.org/jdk/pull/20742/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20742&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339149
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20742.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20742/head:pull/20742

PR: https://git.openjdk.org/jdk/pull/20742