RFR: 7872: Adding artifacts to third-party pom file, which are not getting resolved by maven

[Virag Purnam]

Sometimes below mentioned artifacts are not getting resolved by *mvn p2:site*. 
* jetty-security
* jetty-server
* websocket-core-server
* jetty-xml

These plugins will be missing in jmc -> releng -> third-party -> target ->repository -> plugins.
So adding these to third-party pom file. Adding these artifacts is not creating any effect on existing behavior. These artifacts will be just hosted on local p2 site.

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

Commit messages:
 - 7872: Adding artifacts to third-party pom file, which are not getting resolved by maven
 - 7872: Adding artifacts to third-party pom file, which are not getting resolved by maven

Changes: https://git.openjdk.org/jmc/pull/418/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=418&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7872
  Stats: 37 lines in 2 files changed: 20 ins; 6 del; 11 mod
  Patch: https://git.openjdk.org/jmc/pull/418.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/418/head:pull/418

PR: https://git.openjdk.org/jmc/pull/418