RFR: Make method handle accessor private [v2]

[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.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/jextract/pull/188/files
  - new: https://git.openjdk.org/jextract/pull/188/files/4c77ae51..a7f001ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=188&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=188&range=00-01

  Stats: 18 lines in 1 file changed: 0 ins; 12 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