Fix a redefinition of 'net_gen_cookie' error that was overlooked
when net ns is not configured.

Fixes: f318903c ("bpf: Add netns cookie and enable it for bpf cgroup hooks")
Reported-by: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>