linuxPackages_4_14_hardened: fix eval

Not sure if this is the right thing to do, but apparently we want this
setting for <4.18, so we hard-code it for 4.14 via `mkForce` (previously
this has led to conflicting definitions which is why the hardened 4.14
kernel never seems to have evaluated properly).

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

···
   INET_MPTCP_DIAG   = option no;
 
   # Use -fstack-protector-strong (gcc 4.9+) for best stack canary coverage.
-  CC_STACKPROTECTOR_REGULAR = whenOlder "4.18" no;
+  CC_STACKPROTECTOR_REGULAR = lib.mkForce (whenOlder "4.18" no);
   CC_STACKPROTECTOR_STRONG  = whenOlder "4.18" yes;
 
   # Detect out-of-bound reads/writes and use-after-free