Update periodic_worker-inl.h

Fix: Make this great log library DRD through cleanly test by valgrind --tool=drd.

include/spdlog/details/periodic_worker-inl.h

@@ -16,10 +16,10 @@ SPDLOG_INLINE periodic_worker::~periodic_worker()
     if (worker_thread_.joinable())
     {
         {
-            std::lock_guard<std::mutex> lock(mutex_);
+            std::unique_lock<std::mutex> lock(mutex_);
             active_ = false;
+            cv_.notify_one();
         }
-        cv_.notify_one();
         worker_thread_.join();
     }
 }