Timestamp at capture time

Timestamps are currently added in userspace and likely to be inaccurate: https://github.com/cloudflare/xdpcap/blob/master/cmd/xdpcap/main.go#L120

Switch timestamps to BPF using `bpf_ktime_get_ns` (which is `clock_gettime(CLOCK_MONOTONIC)` and not the actual time, so will need some adjusting in userspace).