evdi: prepend kernel.makeFlags to makeFlags

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

···
 
   buildInputs = [ kernel libdrm ];
 
-  makeFlags = [
+  makeFlags = kernel.makeFlags ++ [
     "KVER=${kernel.modDirVersion}"
     "KDIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
   ];