sync with latest libbpf
sync with latest libbpf

This patch syncs with libbpf v0.0.3.

Signed-off-by: Yonghong Song <e76892bbe783db2f627de1232e139db4de43da60@fb.com>

iovisor/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc