selftests/bpf: Fix compilation failure when CONFIG_NET_FOU!=y

Without CONFIG_NET_FOU bpf selftests are unable to build because of
missing definitions. Add ___local versions of struct bpf_fou_encap and
enum bpf_fou_encap_type to fix the issue.

Signed-off-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240723071031.3389423-1-asavkov@redhat.com