selftests: net: bpf_offload: add 'libbpf_global' to ignored maps

After installing pahole on the CI image we have a new map created
by libbpf. Ignore it otherwise we see:

  Exception: Time out waiting for map counts to stabilize want 2, have 3

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250304233204.1139251-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>