RFR: 8373122: JFR build failure with CDS disabled due to -Werror=unused-function after JDK-8365400

[Hao Sun]

`get_source(const AOTClassLocation* cl, JavaThread* jt)` is only used by `JfrClassDefineEvent::on_restoration()` within an `INCLUDE_CDS` guard.

Add the same `INCLUDE_CDS` guard to mitigate the GCC warning.

Tests: JDK build with CDS disabled passed on both x86_64 and aarch64.
       tier1~3 passed on both x86_64 and aarch64.

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Commit messages:
 - 8373122: JFR build failure with CDS disabled due to -Werror=unused-function after JDK-8365400

Changes: https://git.openjdk.org/jdk/pull/28917/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28917&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373122
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28917.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28917/head:pull/28917

PR: https://git.openjdk.org/jdk/pull/28917