Fix doxygen: add typedef for pfx_for_each

- Resolved doxygen errors (doxygen cant work with fp)
"(warning: argument 'fp' of command @param is not found in the argument list of
pfx_table_for_each_ipv4_record(struct pfx_table *pfx_table, void(fp)(const struct pfx_record *, void *data),
void *data))"