[foreign-jextract] RFR: Fix failing TestClassGeneration

[Jorn Vernee]

Hi,

I forgot to rebase https://github.com/openjdk/panama-foreign/pull/102 this morning and rerun the tests after
https://github.com/openjdk/panama-foreign/pull/100 went in last night, and as a result the new TestClassGeneration test
is failing.

Trivial fix is to add a missing @library entry.

Thanks,
Jorn

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Commit messages:
 - Fix failing TestClassGeneration

Changes: https://git.openjdk.java.net/panama-foreign/pull/104/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/104/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/104.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/104/head:pull/104

PR: https://git.openjdk.java.net/panama-foreign/pull/104