RFR: 8260289: Unable to customize module lists after change JDK-8258411

[Andrew Leonard]

A problem was found downstream with Eclipse OpenJ9 builds whereby since JDK-8258411 they were unable to extend the module lists.
This PR adds a IncludeCustomExtension to the conf/module-loader-map.conf, to enable the module list extension again.

Signed-off-by: Andrew Leonard <anleonar at redhat.com>

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

Commit messages:
 - 8260289: Unable to customize module lists after change JDK-8258411

Changes: https://git.openjdk.java.net/jdk/pull/2219/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2219&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260289
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2219.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2219/head:pull/2219

PR: https://git.openjdk.java.net/jdk/pull/2219