RFR: 8272903: Missing license header in ArenaAllocator.java

[Maurizio Cimadamore]

This small patch adds missing copyright header to a segment allocator implementation class.

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Commit messages:
 - Add missing copyright header

Changes: https://git.openjdk.java.net/jdk/pull/5447/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5447&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272903
  Stats: 25 lines in 1 file changed: 25 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5447.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5447/head:pull/5447

PR: https://git.openjdk.java.net/jdk/pull/5447