RFR: 6951: [core] Missing license.txt in core artifacts

[Guru Hb]

jmc core library (common, flightrecorder*, jdp, and its test) should contain license.txt in its respective jar.

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Commit messages:
 - 6951: [core] Missing license.txt in core artifacts

Changes: https://git.openjdk.java.net/jmc/pull/141/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=141&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6951
  Stats: 26 lines in 2 files changed: 25 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/141.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/141/head:pull/141

PR: https://git.openjdk.java.net/jmc/pull/141