selftests/bpf: Use local type for bpf_fou_encap in test_tunnel_kern

selftests/bpf: Use local type for bpf_fou_encap in test_tunnel_kern

Replace the forward-declared struct bpf_fou_encap with the existing
bpf_fou_encap___local type in the bpf_skb_set_fou_encap and
bpf_skb_get_fou_encap declarations. This removes the need for
the forward declaration and the explicit casts at each call.

Fixes: d17f9b370df6 ("selftests/bpf: Fix compilation failure when CONFIG_NET_FOU!=y")
Signed-off-by: Gregory Bell <grbell@redhat.com>
Link: https://lore.kernel.org/r/20260417154122.2558890-3-grbell@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>