selftests/bpf: Fix unused attribute usage in subprogs_unused test

Correct attribute name is "unused". maybe_unused is a C++17 addition.
This patch fixes compilation warning during selftests compilation.

Fixes: 197afc63 ("libbpf: Don't attempt to load unused subprog as an entry-point BPF program")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201111231215.1779147-1-andrii@kernel.org