Fix log.cpp when compiled with `executorch.enable_et_log=false` (#4358)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4358

Currently, the ASR execution fails with the following error:
`xplat/executorch/runtime/platform/log.cpp:21:25: error: unused variable 'kMaxLogMessageLength' [-Werror,-Wunused-const-variable]` (P1484525874).

This diff marks kMaxLogMessageLength as `maybe_unused`.

Reviewed By: dbort, dvorjackz

Differential Revision: D59641404

fbshipit-source-id: 2a7e324206eb5a31e309dadc4ee3d6c5dc6f68dc