Add a missing define to monitoring/iostats_context_imp.h (#5136)

Summary:
I think when PR https://github.com/facebook/rocksdb/pull/4889 added the `IOSTATS_CPU_TIMER_GUARD` define to this header file, the noop version in the `#else` branch was forgotten.

Not sure if this is common, but on my MacOS machine it breaks my build
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5136

Differential Revision: D14727727

Pulled By: siying

fbshipit-source-id: 1076e56bdbe6ecda01d461b371dabf7f1593a149

monitoring/iostats_context_imp.h

@@ -55,5 +55,6 @@ extern __thread IOStatsContext iostats_context;
 #define IOSTATS(metric) 0
 
 #define IOSTATS_TIMER_GUARD(metric)
+#define IOSTATS_CPU_TIMER_GUARD(metric, env)
 
 #endif  // ROCKSDB_SUPPORT_THREAD_LOCAL