pppd: fix build with 3.5+ kernel headers

Fixes http://autobuild.buildroot.net/results/8a9369fece696e012249b865708ffee533c5318a

Pppd bundles some, but not all needed kernel headers. In Linux 3.5 (and
before that 2.6.35) if_pppol2tp.h was changed, so the embedded copy was no
longer compatible.

if_pppol2tp.h has been part of the exported kernel headers since 2.6.23, so
simply drop the bundled copy and use the kernel headers instead, similar
to how it is done in Gentoo:

https://bugs.gentoo.org/show_bug.cgi?id=427684



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>