native: fixed OS X includes in periph/timer.c

Consider the following configuration:

    nib prefix
    2001:16b8:4569:88fc::/62 dev #7  expires 7081 sec deprecates 3481 sec
    2001:16b8:4569:88fe::/63 dev #6

If `_on_link()` stops at the first match, a packet received from #7 with a
destination in the downstream subnet in #6 would always be sent back via #7
if this happens to be the first entry in the list.

Instead, consider all prefixes and return the one that is the closest match.