configure.in: Look for ip_vs.h in the kernel sources.

configure.in: Look for ip_vs.h in the kernel sources.

Developers,

I am writing an updated ebuild for gentoo and I made this patch that allows
ip_vs.h to be located and the ipvs module to build. Normally I would include
this with the ebuild, but I thought it may be beneficial to send it to you.
Please review it and let me know what you think. I am still inexperienced with
autotools so the patch may not be well put together. If you would consider
applying it or a similar patch, please let me know so I can adjust the ebuild
accordingly.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>