RFR: 8266757: Add entry for jdk-12_doc-all.zip to gradle/verification-metadata.xml

[Kevin Rushforth]

As indicated in JBS this PR adds a missing entry for the JDK 12 API docs zip bundle for javadoc offline processing. This missing entry is causing a build failure on my system (which will soon propagate to our CI builds).

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Commit messages:
 - 8266757: Add entry for jdk-12_doc-all.zip to gradle/verification-metadata.xml

Changes: https://git.openjdk.java.net/jfx/pull/489/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=489&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266757
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/489.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/489/head:pull/489

PR: https://git.openjdk.java.net/jfx/pull/489