mblaze: 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/applications/networking/mailreaders/mblaze/default.nix

···
   version = "1.2";
 
   nativeBuildInputs = [ installShellFiles makeWrapper ];
-  buildInputs = [ ruby ] ++ lib.optionals stdenv.isDarwin [ libiconv ];
+  buildInputs = [ libiconv ruby ];
 
   src = fetchFromGitHub {
     owner = "leahneukirchen";