Added msteen to the list of maintainers.

lib/maintainers.nix

···
   mornfall = "Petr Ročkai <[e-mail]>";
   MP2E = "Cray Elliott <[e-mail]>";
   msackman = "Matthew Sackman <[e-mail]>";
+  msteen = "Matthijs Steen <[e-mail]>";
   mtreskin = "Max Treskin <[e-mail]>";
   mudri = "James Wood <[e-mail]>";
   muflax = "Stefan Dorn <[e-mail]>";