libbpf: Export bpf_object__prepare symbol

Add missing LIBBPF_API macro for bpf_object__prepare function to enable
its export. libbpf.map had bpf_object__prepare already listed.

Fixes: 1315c28e ("libbpf: Split bpf object load into prepare/load")
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20250819215119.37795-1-mykyta.yatsenko5@gmail.com