pppd: better fix for the Linux 4.8 kernel headers issue

pppd: better fix for the Linux 4.8 kernel headers issue

Commit 035567b2793f3649e2ddc6cd34b75dd643f47d73 solved the build issue
of pppd with Linux 4.8 kernel headers, but in the process broke the
build with older kernel headers such as 3.10.

This commit takes a different approach, by moving the <netinet/in.h>
include above linux header includes.

Linux uapi headers try to avoid redeclaring things from libc via
linux/libc-compat.h. However that only works if the libc header is
included first.

Signed-off-by: Stefan Nickl <Stefan.Nickl@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>