Get struct ether_addr directly from <net/ethernet.h> rather than pulling in lots of unrelated junk from <net/if.h> and <net/if_ether.h>. These functions still aren't prototyped anywhere (but should be in <net/ethernet.h>---got that, Bill?).
head/lib/libc/net/ether_addr.c
head/lib/libc/net/ethers.3