linuxHeaders: use elf-header from build packages

Splicing does not work correctly here, so elf-header was being
pulled from the host packages. It must be pulled from the build
packages since it is used by the HOSTCC for build-time tools.

Note that using buildPackages here causes infinite recursion.

Fixes darwin -> linux-mips cross-compilation of linuxHeaders.

pkgs/top-level/all-packages.nix

···
   lkl = callPackage ../applications/virtualization/lkl { };
   lklWithFirewall = callPackage ../applications/virtualization/lkl { firewallSupport = true; };
 
-  inherit (callPackages ../os-specific/linux/kernel-headers { })
+  inherit (callPackages ../os-specific/linux/kernel-headers { inherit (pkgsBuildBuild) elf-header; })
     linuxHeaders makeLinuxHeaders;
 
   klibc = callPackage ../os-specific/linux/klibc { };