Cast unused variable to void

Fixes -wunused-variable warning

PiperOrigin-RevId: 478886333
Change-Id: Ib84def2ddaf6a6860606e78603c2c25865ce2814

absl/log/internal/log_sink_set.cc

@@ -69,6 +69,7 @@ bool& ThreadIsLoggingStatus() {
     }
     }
     return true;
     return true;
   }();
   }();
+  (void)unused;  // Fixes -wunused-variable warning
   bool* thread_is_logging_ptr =
   bool* thread_is_logging_ptr =
       reinterpret_cast<bool*>(pthread_getspecific(thread_is_logging_key));
       reinterpret_cast<bool*>(pthread_getspecific(thread_is_logging_key));