RFR: 8326433: Make libjdwp and libjava closeDescriptors() as static function

[Jiangli Zhou]

Please help review this trivial fix for resolving `ld: error: duplicate symbol: closeDescriptors` when static linking with both libjdwp and libjava, thanks.

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Commit messages:
 - Make closeDescriptors() as static function in src/java.base/unix/native/libjava/childproc.c and src/jdk.jdwp.agent/unix/native/libjdwp/exec_md.c.

Changes: https://git.openjdk.org/jdk/pull/18013/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18013&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326433
  Stats: 3 lines in 3 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18013.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18013/head:pull/18013

PR: https://git.openjdk.org/jdk/pull/18013