Fixed warning

include/spdlog/stopwatch.h

@@ -36,8 +36,10 @@ public:
     stopwatch()
         : start_tp_{clock::now()} {}
 
+    [[nodiscard]]
     std::chrono::duration<double> elapsed() const { return std::chrono::duration<double>(clock::now() - start_tp_); }
 
+    [[nodiscard]]
     std::chrono::milliseconds elapsed_ms() const {
         return std::chrono::duration_cast<std::chrono::milliseconds>(clock::now() - start_tp_);
     }