Merge pull request #2336 from aengusjiang/v1.x

[issues/2332]clean code, clean up the warning

src/async.cpp

@@ -10,4 +10,7 @@
 #include <spdlog/details/periodic_worker-inl.h>
 #include <spdlog/details/thread_pool-inl.h>
 
-template class SPDLOG_API spdlog::details::mpmc_blocking_queue<spdlog::details::async_msg>;
+/*
+ * comment the declaration for the warning: type attributes ignored after type is already defined [-Wattributes]
+ */
+//template class SPDLOG_API spdlog::details::mpmc_blocking_queue<spdlog::details::async_msg>;