fix linking without thread status support

When compiling with `-DNROCKSDB_THREAD_STATUS`, some functions in
ThreadStatusUtil are declared but their definition is missing. Their
definitions are only compiled when not defining `NROCKSDB_THREAD_STATUS`.
This causes problems on linking, when the linker cannot find the
definitions of
- ThreadStatusUtil::GetThreadOperation
- ThreadStatusUtil::SetEnableTracking

This PR fixes it by adding stubs for these functions in case
`NROCKSDB_THREAD_STATUS` is defined.

monitoring/thread_status_util.cc

@@ -175,8 +175,14 @@ bool ThreadStatusUtil::MaybeInitThreadLocalUpdater(const Env* /*env*/) {
   return false;
 }
 
+void ThreadStatusUtil::SetEnableTracking(bool /*enable_tracking*/) {}
+
 void ThreadStatusUtil::SetColumnFamily(const ColumnFamilyData* /*cfd*/) {}
 
+ThreadStatus::OperationType ThreadStatusUtil::GetThreadOperation() {
+  return ThreadStatus::OperationType::OP_UNKNOWN;
+}
+
 void ThreadStatusUtil::SetThreadOperation(ThreadStatus::OperationType /*op*/) {}
 
 void ThreadStatusUtil::SetThreadOperationProperty(int /*code*/,