selftests/bpf: Use bpf_tracing_net.h in bpf_cubic

This patch uses bpf_tracing_net.h (i.e. vmlinux.h) in bpf_cubic.
This will allow to retire the bpf_tcp_helpers.h and consolidate
tcp-cc tests to vmlinux.h.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240509175026.3423614-7-martin.lau@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>