It's part of a series of functions that retrieve IP & Mac addresses from the phone.
strcpy(temp, (char *)ether_ntoa((const struct ether_addr *)LLADDR(sdl)));
As I read it, the error message isn't claiming that the function is missing, only that you don't include its declaration. (I don't know that it exists, only that the message has a different complaint.)
In case it helps,
man ether_ntoa tells me:
#include <sys/types.h> #include <sys/socket.h> #include <net/ethernet.h>