Make Property cast operator inline

src/config/Property.h

@@ -49,7 +49,7 @@ namespace Feather
         T GetValue() { return value; }
 
         // Property<T> can be automatically cast to T
-        operator T() const { return value; }
+        inline operator T() const { return value; }
 
         inline string ToString() const { return std::to_string(value); }