systemd: get linuxHeaders from stdenv

Building against anything other than the headers used to build libc
doesn't really make any sense.

pkgs/os-specific/linux/systemd/default.nix

···
 { stdenv, fetchFromGitHub, pkgconfig, intltool, gperf, libcap, kmod
 , zlib, xz, pam, acl, cryptsetup, libuuid, m4, utillinux, libffi
 , glib, kbd, libxslt, coreutils, libgcrypt, libgpgerror, libapparmor, audit, lz4
-, kexectools, libmicrohttpd, linuxHeaders, libseccomp, iptables
+, kexectools, libmicrohttpd, linuxHeaders ? stdenv.cc.libc.linuxHeaders, libseccomp
+, iptables
 , autoreconfHook, gettext, docbook_xsl, docbook_xml_dtd_42, docbook_xml_dtd_45
 , enableKDbus ? false
 }:

pkgs/top-level/all-packages.nix

···
   sysstat = callPackage ../os-specific/linux/sysstat { };
 
   systemd = callPackage ../os-specific/linux/systemd {
-    linuxHeaders = linuxHeaders_3_18;
     utillinux = utillinuxMinimal; # break the cyclic dependency
   }
     // {