ncdu: pie for darwin builds

Fix broken darwin compilation. The issue started after the 2.2.2 ->
2.3 update (0b6d395909).

`ncdu` defaults pie to false while `zig` requires it to be true for
darwin.

pkgs/tools/misc/ncdu/default.nix

@@ -4,6 +4,7 @@
 , ncurses
 , zig_0_11
 , installShellFiles
+, pie ? stdenv.isDarwin
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -24,6 +25,8 @@ stdenv.mkDerivation (finalAttrs: {
     ncurses
   ];
 
+  zigBuildFlags = lib.optional pie "-Dpie=true";
+
   postInstall = ''
     installManPage ncdu.1
   '';