arc_unpacker: 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/tools/archivers/arc_unpacker/default.nix

@@ -13,8 +13,7 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = [ cmake makeWrapper catch2 ];
-  buildInputs = [ boost libpng libjpeg zlib openssl libwebp ]
-    ++ lib.optionals stdenv.isDarwin [ libiconv ];
+  buildInputs = [ boost libiconv libjpeg libpng libwebp openssl zlib ];
 
   postPatch = ''
     cp ${catch2}/include/catch2/catch.hpp tests/test_support/catch.h