busybox: fix static build

The static build fails with undefined references to __memcpy_chk when
the fortify hardening is enabled.

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

@@ -33,7 +33,7 @@ stdenv.mkDerivation rec {
     sha256 = "16ii9sqracvh2r1gfzhmlypl269nnbkpvrwa7270k35d3bigk9h5";
   };
 
-  hardeningDisable = [ "format" ];
+  hardeningDisable = [ "format" ] ++ lib.optional enableStatic [ "fortify" ];
 
   patches = [ ./busybox-in-store.patch ];