nixos-option: set platforms.all

only built on linux currently but it is used on non-linux platforms e.g. darwin with home-manager

pkgs/tools/nix/nixos-option/default.nix

···
   meta = with lib; {
     license = licenses.lgpl2Plus;
     maintainers = with maintainers; [ chkno ];
+    platforms = platforms.all;
   };
 }