nixos/flashprog: Add Felix Singer as maintainer

Signed-off-by: Felix Singer <[e-mail]>

nixos/modules/programs/flashprog.nix

···
     services.udev.packages = [ cfg.package ];
     environment.systemPackages = [ cfg.package ];
   };
+
+  meta.maintainers = with lib.maintainers; [ felixsinger ];
 }