selftests/bpf: add subprog to BPF object file with no entry programs

selftests/bpf: add subprog to BPF object file with no entry programs

Add a subprogram to BPF object file that otherwise has no entry BPF
programs to validate that libbpf can still load this correctly.

Until this was fixed, user could expect this very confusing error message:

  libbpf: prog 'dangling_subprog': missing BPF prog type, check ELF section name '.text'
  libbpf: prog 'dangling_subprog': failed to load: -22
  libbpf: failed to load object 'struct_ops_detach'
  libbpf: failed to load BPF skeleton 'struct_ops_detach': -22

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241010211731.4121837-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>