RFR: 8329958: jdk22 win x86 make fails: downcallLinker.cpp(36) redefinition

[David Holmes]

Trivial fix to add JNICALL to the function declaration.

This will be backported to JDK 22.

Testing:
 - tier1 sanity builds

Thanks

-------------

Commit messages:
 - 8329958: jdk22 win x86 make fails: downcallLinker.cpp(36) redefinition

Changes: https://git.openjdk.org/jdk/pull/19406/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19406&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329958
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19406.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19406/head:pull/19406

PR: https://git.openjdk.org/jdk/pull/19406