bpftrace was using an approach different from bcc to calculate the
offset of an USDT probe. We were using only `bcc_usdt_get_location`,
whereas bcc is using a combination of `bcc_usdt_get_location` and
`bcc_resolve_symname`. Since we already use `bcc_resolve_symname` in
`AttachedProbe::offset()`, we just need to call this method to get the
same behavior from bcc.

Fixes: https://github.com/iovisor/bpftrace/issues/60