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

Summary:
Pull Request resolved: #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`.

Differential Revision: D59641404

runtime/platform/log.cpp

@@ -18,7 +18,7 @@ namespace executor {
 namespace internal {
 
 /// Maximum length of a log message.
-static constexpr size_t kMaxLogMessageLength = 256;
+__ET_UNUSED static constexpr size_t kMaxLogMessageLength = 256;
 
 /**
  * Get the current timestamp to construct a log event.