buildroot

libpcap: bump to version 1.8.1

Drop upstream patch.

Also remove ac_cv_linux_vers caching since this check was remove in upstream
commit f3e011b18c55ae.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>