include/uapi/linux/if_pppox.h: include linux/in.h and linux/in6.h

Fixes userspace compilation errors:

error: field ‘addr’ has incomplete type
 struct sockaddr_in addr; /* IP address and port to send to */

error: field ‘addr’ has incomplete type
 struct sockaddr_in6 addr; /* IP address and port to send to */

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>