Restore init thread-local var in mdc.h, not use reference-init

include/spdlog/mdc.h

@@ -38,7 +38,7 @@ public:
     static void clear() { get_context().clear(); }
 
     static mdc_map_t &get_context() {
-        static thread_local mdc_map_t& context = *new mdc_map_t;
+        static thread_local mdc_map_t context;
         return context;
     }
 };