wavpack: 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/wavpack/default.nix

@@ -7,7 +7,7 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   nativeBuildInputs = [ autoreconfHook ];
-  buildInputs = lib.optional stdenv.isDarwin libiconv;
+  buildInputs = [ libiconv ];
 
   # autogen.sh:9
   preAutoreconf = "cp ${gettext}/share/gettext/config.rpath .";