veikk-linux-driver: prepend kernel.makeFlags to makeFlags

pkgs/os-specific/linux/veikk-linux-driver/default.nix

···
 
   buildInputs = [ kernel ];
 
-  buildPhase = ''
-    make BUILD_DIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build
-  '';
+  makeFlags = kernel.makeFlags ++ [
+    "BUILD_DIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
+  ];
 
   installPhase = ''
     mkdir -p $out/lib/modules/${kernel.modDirVersion}/kernel/drivers/veikk