RFR: 7903856: apidiff: 7903837 breaks some builds

[Jonathan Gibbons]

Please review a small but important fix for an issue handling EQUINOX_JAR in some build environments.

The underlying issue is that EQUINOX_JAR may or may not be required, depending on the configuration for DAISYDIFF.

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

Commit messages:
 - CODETOOLS-7903856: apidiff: 7903837 breaks some builds

Changes: https://git.openjdk.org/apidiff/pull/22/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=22&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903856
  Stats: 10 lines in 2 files changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/apidiff/pull/22.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/22/head:pull/22

PR: https://git.openjdk.org/apidiff/pull/22