selftests: bpf: fix -Wformat-invalid-specifier for bpf_obj_id.c

Use standard C99 %zu for sizeof, not GCC's custom %Zu:
bpf_obj_id.c:76:48: warning: invalid conversion specifier 'Z'

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>