RFR: 8343513: Forward declare Thread in mutexLocker.hpp

[Johan Sjölen]

Hi,

Let's explicitly forward declare `Thread` in `mutexLocker.hpp` instead of including `thread.hpp`. Previously we got the forward declaration from `memory/allocation.hpp`, but let's be explicit about forward declarations being used when we can.

Thanks

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Commit messages:
 - Forward declare class Thread

Changes: https://git.openjdk.org/jdk/pull/21868/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21868&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343513
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21868.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21868/head:pull/21868

PR: https://git.openjdk.org/jdk/pull/21868