build: ship a include/ directory

build: ship a include/ directory

Some (old) distributions are shipping outdated kernel headers. Some
oddities were detected in `configure` and corrected in `compat.h` but
this is cumbersome. Like `iproute`, we now ship a "complete" include/
directory containing the missing bits. It comes from a 3.6.9
kernel. Using too recent headers with too old kernel should not be a
problem. Those headers are just definition and there was not guarantee
that the kernel used to compile was the one which will be used to run.