pass log_time param by value

include/spdlog/logger.h

@@ -147,7 +147,7 @@ public:
         log(loc, lvl, string_view_t{msg});
     }
 
-    void log(log_clock::time_point &log_time, source_loc loc, level::level_enum lvl, string_view_t msg)
+    void log(log_clock::time_point log_time, source_loc loc, level::level_enum lvl, string_view_t msg)
     {
         bool log_enabled = should_log(lvl);
         bool traceback_enabled = tracer_.enabled();