linuxPackages.bbswitch: use kernel's make flags

We should be using whatever makeflags were used to build the kernel
itself, otherwise clang-built kernels cannot use the module.

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

@@ -36,6 +36,8 @@ stdenv.mkDerivation {
       --replace "/lib/modules" "${kernel.dev}/lib/modules"
   '';
 
+  makeFlags = kernel.makeFlags;
+
   installPhase = ''
     mkdir -p $out/lib/modules/${kernel.modDirVersion}/misc
     cp bbswitch.ko $out/lib/modules/${kernel.modDirVersion}/misc