selftests/bpf: Skip lsm_cgroup when we don't have trampolines

With arch_prepare_bpf_trampoline removed on x86:

  [...]
  #98/1    lsm_cgroup/functional:SKIP
  #98      lsm_cgroup:SKIP
  Summary: 1/0 PASSED, 1 SKIPPED, 0 FAILED

Fixes: dca85aac

 ("selftests/bpf: lsm_cgroup functional test")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20220630224203.512815-1-sdf@google.com