The file itself is just build specific information of postfix on your system.. This symlink was made when you installed postfix.
The only reason you get this warning is that this symlink points to a file outside of your /etc/postfix directory. It is flagged with a warning because it could indicate a security issue if you didn’t expect it to be there. in this case the symlink is created by your OS's package manager so it’s fine.
You don’t fix it, or rather you probably shouldn’t bother, it’s important to note that this is just a warning, not an error