sync to latest libbpf
sync to latest libbpf

sync to latest libbpf (libbpf release v0.0.7) to
prepare bcc release.

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

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