RFR: 8326714: Make file-local functions static in src/java.base/unix/native/libjava/childproc.c

[Jiangli Zhou]

Please help review this trivial change. This was branched from https://github.com/openjdk/jdk/pull/18013, based on discussion with @plummercj in https://github.com/openjdk/jdk/pull/18013 comments. Thanks

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Commit messages:
 - 8326714: Make file-local functions static in src/java.base/unix/native/libjava/childproc.c

Changes: https://git.openjdk.org/jdk/pull/18019/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18019&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326714
  Stats: 20 lines in 2 files changed: 0 ins; 13 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/18019.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18019/head:pull/18019

PR: https://git.openjdk.org/jdk/pull/18019