uapi: netfilter_arp: use __u8 instead of u_int8_t

Similarly, the u_int8_t type is non-standard and not defined.  Change
it to use __u8 like the rest of the netfilter headers.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>