RFR: 6941: build.sh does not terminate jetty after executing '--packageJmc'

[Patrick Reinhart]

stopping jetty instance using kill command working on Mac & Linux

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Commit messages:
 - 6941: stopping jetty instance using kill command working on Mac & Linux

Changes: https://git.openjdk.java.net/jmc/pull/130/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=130&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6941
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/130.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/130/head:pull/130

PR: https://git.openjdk.java.net/jmc/pull/130