[foreign-jextract] RFR: Add libclang dependency to docs jib task

[Maurizio Cimadamore]

This simple patch adds the dependency on libclang to the javadoc jib task.

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Commit messages:
 - Add libclang dependency to docs task

Changes: https://git.openjdk.java.net/panama-foreign/pull/420/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=420&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/420.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/420/head:pull/420

PR: https://git.openjdk.java.net/panama-foreign/pull/420