RFR: 8307969: [zgc] Missing includes in gc/z/zTracer.cpp

[Jie Fu]

This fixes the build broken with `--with-jvm-features=-jfr`.
Thanks.

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Commit messages:
 - 8307969: [zgc] Missing includes in gc/z/zTracer.cpp

Changes: https://git.openjdk.org/jdk/pull/13948/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13948&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307969
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13948.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13948/head:pull/13948

PR: https://git.openjdk.org/jdk/pull/13948