Build: Require fontconfig on macos

[gigamonster256]

I was working on getting the nix version of ladybird working on macos and noticed that as of https://github.com/LadybirdBrowser/ladybird/commit/dd1b8de671747f0a5efc84b77933e13208136507, fontconfig is required on macos but the fontconfig cmake file was not updated.