sd-pppoe: include ppp_defs.h

On older kernels before this patch:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=e8b671460410c8fd996c8a1c228b718c547cc236
ppp-ioctl.h did not pull in ppp_defs.h which results in build errors