libdatrie: depend on libiconv unconditionally

libiconv is already defined per-platform.  The actual libiconv library
won't be built on platforms like Linux where it doesn't need to be, so
there's no need to maintain a separate platform list here.

Required to build for FreeBSD.

pkgs/development/libraries/libdatrie/default.nix

@@ -22,7 +22,7 @@ stdenv.mkDerivation rec {
     installShellFiles
   ];
 
-  buildInputs = lib.optional stdenv.isDarwin libiconv;
+  buildInputs = [ libiconv ];
 
   preAutoreconf = let
     reports = "https://github.com/tlwg/libdatrie/issues";