linuxKernel.kernels.linux_5_13_hardened: fix build

BTF cannot be enabled at the same time as the RANDSTRUCT GCC plugin, so
we need to mark it as optional. Alas.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -40,7 +40,7 @@ let
         (whenOlder "5.2" (if (features.debug or false) then yes else no))
         (whenAtLeast "5.2" yes)
       ];
-      DEBUG_INFO_BTF            = whenAtLeast "5.2" yes;
+      DEBUG_INFO_BTF            = whenAtLeast "5.2" (option yes);
       DEBUG_KERNEL              = yes;
       DEBUG_DEVRES              = no;
       DYNAMIC_DEBUG             = yes;