Fixes unused var (open-telemetry#3397)

* Fixes unused var

* Ignore IWYU in example

Author: owent

examples/otlp/http_instrumented_main.cc

@@ -4,7 +4,7 @@
 #include <chrono>
 #include <iostream>
 #include <memory>
-#include <mutex>
+#include <mutex>  // IWYU pragma: keep
 #include <string>
 #include <utility>
 
@@ -58,10 +58,10 @@
 namespace
 {
 
-std::mutex serialize;
-
 #ifdef ENABLE_THREAD_INSTRUMENTATION_PREVIEW
 
+std::mutex serialize;
+
 /**
  The purpose of MyThreadInstrumentation is to demonstrate
  how notifications are delivered to the application.