fix compiler errors with -DNPERF_CONTEXT (#6642)

Summary:
as titled
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6642

Test Plan:
```
$ EXTRA_CXXFLAGS="-DNPERF_CONTEXT" DEBUG_LEVEL=0 make -j48 db_bench
```

Reviewed By: riversand963

Differential Revision: D20842313

Pulled By: ajkr

fbshipit-source-id: a830cad312ca681591f06749242279503b101df2

monitoring/instrumented_mutex.cc

@@ -10,6 +10,7 @@
 
 namespace ROCKSDB_NAMESPACE {
 namespace {
+#ifndef NPERF_CONTEXT
 Statistics* stats_for_report(Env* env, Statistics* stats) {
   if (env != nullptr && stats != nullptr &&
       stats->get_stats_level() > kExceptTimeForMutex) {
@@ -18,6 +19,7 @@ Statistics* stats_for_report(Env* env, Statistics* stats) {
     return nullptr;
   }
 }
+#endif  // NPERF_CONTEXT
 }  // namespace
 
 void InstrumentedMutex::Lock() {

monitoring/perf_context.cc

@@ -38,7 +38,9 @@ PerfContext::~PerfContext() {
 }
 
 PerfContext::PerfContext(const PerfContext& other) {
-#ifndef NPERF_CONTEXT
+#ifdef NPERF_CONTEXT
+  (void)other;
+#else
   user_key_comparison_count = other.user_key_comparison_count;
   block_cache_hit_count = other.block_cache_hit_count;
   block_read_count = other.block_read_count;
@@ -133,7 +135,9 @@ PerfContext::PerfContext(const PerfContext& other) {
 }
 
 PerfContext::PerfContext(PerfContext&& other) noexcept {
-#ifndef NPERF_CONTEXT
+#ifdef NPERF_CONTEXT
+  (void)other;
+#else
   user_key_comparison_count = other.user_key_comparison_count;
   block_cache_hit_count = other.block_cache_hit_count;
   block_read_count = other.block_read_count;
@@ -230,7 +234,9 @@ PerfContext::PerfContext(PerfContext&& other) noexcept {
 // TODO(Zhongyi): reduce code duplication between copy constructor and
 // assignment operator
 PerfContext& PerfContext::operator=(const PerfContext& other) {
-#ifndef NPERF_CONTEXT
+#ifdef NPERF_CONTEXT
+  (void)other;
+#else
   user_key_comparison_count = other.user_key_comparison_count;
   block_cache_hit_count = other.block_cache_hit_count;
   block_read_count = other.block_read_count;
@@ -443,6 +449,7 @@ void PerfContextByLevel::Reset() {
 
 std::string PerfContext::ToString(bool exclude_zero_counters) const {
 #ifdef NPERF_CONTEXT
+  (void)exclude_zero_counters;
   return "";
 #else
   std::ostringstream ss;