ptpd2: explicitly point to pcap-config

Otherwise it will search in the usual places, and if the host has
it it will try to build against that one, resulting in failure. Fixes:
http://autobuild.buildroot.net/results/b39/b399ee830de587e3302f86ac0caadd2607c6c43c/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>