bpf, net: Fix build issue when net ns not configured

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: kbuild test robot <lkp@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>