RFR: 8351821: VMManagementImpl.c avoid switching off warnings

[Matthias Baesken]

We switched off unused-variable warnings for the file VMManagementImpl.c, this should be avoided.
This came up in [JDK-8351542](https://bugs.openjdk.org/browse/JDK-8351542) ,

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Commit messages:
 - JDK-8351821

Changes: https://git.openjdk.org/jdk/pull/24052/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24052&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351821
  Stats: 8 lines in 2 files changed: 0 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24052.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24052/head:pull/24052

PR: https://git.openjdk.org/jdk/pull/24052