libbpf: Provide GELF_ST_VISIBILITY() define for older libelf

Where that macro isn't available.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/YJaspEh0qZr4LYOc@kernel.org