Add musl-fts for bits that need it.

pkgs/os-specific/linux/musl/fts.nix

···
+{ stdenv, fetchFromGitHub, autoreconfHook, pkgconfig }:
+
+stdenv.mkDerivation rec {
+  name = "musl-fts-${version}";
+  version = "2017-01-13";
+  src = fetchFromGitHub {
+    owner = "pullmoll";
+    repo = "musl-fts";
+    rev = "0bde52df588e8969879a2cae51c3a4774ec62472";
+    sha256 = "1q8cpzisziysrs08b89wj0rm4p6dsyl177cclpfa0f7spjm3jg03";
+  };
+
+  nativeBuildInputs = [ autoreconfHook pkgconfig ];
+}

pkgs/top-level/all-packages.nix

···
   multipath-tools = callPackage ../os-specific/linux/multipath-tools { };
 
   musl = callPackage ../os-specific/linux/musl { };
+  musl-fts = callPackage ../os-specific/linux/musl/fts.nix { };
 
   nettools = callPackage ../os-specific/linux/net-tools { };