RFR: Fix compilation failure after #229

[Jorn Vernee]

Changes in https://github.com/openjdk/jextract/pull/229 result in a compilation failure.

A cast that looks redundant was removed, but is actually needed for the code to compile.

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Commit messages:
 - Fix compilation failure

Changes: https://git.openjdk.org/jextract/pull/230/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=230&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/230.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/230/head:pull/230

PR: https://git.openjdk.org/jextract/pull/230