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

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

The patch ppp-fix-building-with-linux-4.8.patch tries to fix build error
with 4.8 or newer linux headers, but it would break building with kernel
< 4.8. There is a better solution to fix this issue in upstream.
Backport the upstream patch and replace the old one.

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>