headers, pppox: Add missing #include to <linux/if_pppox.h>

<linux/if_ppox.h> uses ETH_ALEN, defined in <linux/if_ether.h>.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>