RFR: 8163: Fix update-site not working

[Hélios GILLES]

Fix : https://bugs.openjdk.org/browse/JMC-8163

Missing rjmx.common plugin, as well as the fireplace plugins in
jmc.feature.core/feature.xml

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Commit messages:
 - 8163: Fix update-site not working

Changes: https://git.openjdk.org/jmc/pull/545/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=545&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8163
  Stats: 35 lines in 1 file changed: 34 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jmc/pull/545.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/545/head:pull/545

PR: https://git.openjdk.org/jmc/pull/545