include/uapi/linux/if_pppox.h: include linux/if.h

Fixes userspace compilation error:

error: ‘IFNAMSIZ’ undeclared here (not in a function)

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