RFR: 8298055: AArch64: fastdebug build fails after JDK-8247645

[Fei Yang]

Please review this trivial change fixing a fastdebug build failure due to warnings being treated as errors after JDK-8247645.

Testing: fastdebug build fine with this change on linux-aarch64 platform.

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Commit messages:
 - 8298055: AArch64: fastdebug build fails after JDK-8247645

Changes: https://git.openjdk.org/jdk/pull/11496/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11496&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298055
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11496.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11496/head:pull/11496

PR: https://git.openjdk.org/jdk/pull/11496