Update mdc.h

include/spdlog/mdc.h

@@ -1,9 +1,12 @@
 #pragma once
 
 #include <map>
+#include <string>
+
+#include <spdlog/common.h>
 
 namespace spdlog {
-    class mdc {
+    class SPDLOG_API mdc {
     public:
         using mdc_map_t = std::map<std::string, std::string>;