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: 197afc6 ("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

Author: anakryiko

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

@@ -4,12 +4,12 @@
 
 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;
 }