Signed-off-by: Rosen Penev <rosenp@gmail.com>

- Ship a `net/ppp_defs.h` replacement header since musl does not provide one
   but `pppd/pppd.h` provided by pppd is needing it.
 - Add missing `sys/types.h` includes

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>