libbpf: Correct debug message in btf__load_vmlinux_btf

In the function btf__load_vmlinux_btf, the debug message incorrectly
refers to 'path' instead of 'sysfs_btf_path'.

Signed-off-by: Chen Shen <peterchenshen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240302062218.3587-1-peterchenshen@gmail.com