linuxPackages.v4l2loopback: fix compilation with linux-4.14

pkgs/os-specific/linux/v4l2loopback/default.nix

···
     export PATH=${kmod}/sbin:$PATH
   '';
 
+  nativeBuildInputs = kernel.moduleBuildDependencies;
   buildInputs = [ kmod ];
 
   makeFlags = [