libbpf: Suppress compiler warning when using SEC() macro with externs

When used on externs SEC() macro will trigger compilation warning about
inapplicable `__attribute__((used))`. That's expected for extern declarations,
so suppress it with the corresponding _Pragma.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-4-andrii@kernel.org