pppoe: include netinet/in.h before linux/in.h

This fixes builds with newer kernels. Basically, <netinet/in.h> needs to be
included before <linux/in.h> otherwise the earlier, unaware of the latter,
tries to redefine symbols and structures. Also, <linux/if_pppox.h> doesn't work
alone anymore, since it pulls the headers in the wrong order, so we better
include <netinet/in.h> early.