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: 197afc631413 ("libbpf: Don't attempt to load unused subprog as an entry-point BPF program")
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/test_subprogs_unused.c

···
 
 const char LICENSE[] SEC("license") = "GPL";
 
-__attribute__((maybe_unused)) __noinline int unused1(int x)
+__attribute__((unused)) __noinline int unused1(int x)
 {
 	return x + 1;
 }
 
-static __attribute__((maybe_unused)) __noinline int unused2(int x)
+static __attribute__((unused)) __noinline int unused2(int x)
 {
 	return x + 2;
 }