add build time dependency to static lib

the header only fmt lib is used to show this

include/greeter/greeter.h

@@ -26,6 +26,9 @@ namespace greeter {
      * @return a string containing the greeting
      */
     std::string greet(LanguageCode lang = LanguageCode::EN) const;
+
+    /// @brief Return an iso date string
+    std::string getIsoDate() const;
   };
 
 }  // namespace greeter