RFR: Make method handle accessor private

[Maurizio Cimadamore]

In my previous PR, I forgot to also make the method handle accessor (for native downcalls) `private`.

This was discovered when manually eyeballing at jextracted code for windows.h.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/188/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=188&range=00
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/188.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/188/head:pull/188

PR: https://git.openjdk.org/jextract/pull/188