lttng-modules: use kernel.moduleBuildDependencies

Instead of explicitly depending on libelf, use
kernel.moduleBuildDependencies which was introduced in 1e77d0b975ad
("kernel 4.14 require libelf to compile modules.").

pkgs/os-specific/linux/lttng-modules/default.nix

···
-{ stdenv, fetchurl, kernel, libelf }:
+{ stdenv, fetchurl, kernel }:
 
 stdenv.mkDerivation rec {
   pname = "lttng-modules-${version}";
···
     sha256 = "07rs01zwr4bmjamplix5qz1c6mb6wdawb68vyn0w6wx68ppbpnxq";
   };
 
-  buildInputs = [ libelf ];
+  buildInputs = kernel.moduleBuildDependencies;
 
   hardeningDisable = [ "pic" ];