Fix template issue

include/spdlog/details/registry.h

@@ -180,13 +180,14 @@ private:
     std::function<void()> _worker_warmup_cb = nullptr;
     std::chrono::milliseconds _flush_interval_ms;
 };
+
+template<class Mutex>
+std::shared_ptr<registry_t<Mutex>> registry_t<Mutex>::_instance = nullptr;
+
 #ifdef SPDLOG_NO_REGISTRY_MUTEX
 typedef registry_t<spdlog::details::null_mutex> registry;
 #   else
 typedef registry_t<std::mutex> registry;
 #endif //SPDLOG_NO_REGISTRY_MUTEX
-
-std::shared_ptr<registry> registry::_instance = nullptr;
-
 }
 }