[foreign-jextract] RFR: 8252705: private static final constants lead to compilation errors

[Athijegannathan Sundararajan]

constants are made package static final instead of private static final

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Commit messages:
 - 8252705: private static final constants lead to compilation errors

Changes: https://git.openjdk.java.net/panama-foreign/pull/303/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=303&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252705
  Stats: 8 lines in 1 file changed: 0 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/303.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/303/head:pull/303

PR: https://git.openjdk.java.net/panama-foreign/pull/303