setserial: fix cross compilation

The setserial derivation uses nroff at compile time, so groff should
be in nativeBuildInputs rather than buildInputs.

pkgs/tools/system/setserial/default.nix

···
     sha256 = "0jkrnn3i8gbsl48k3civjmvxyv9rbm1qjha2cf2macdc439qfi3y";
   };
 
-  buildInputs = [ groff ];
+  nativeBuildInputs = [ groff ];
 
   installFlags = [ "DESTDIR=$(out)" ];