ptpd2: needs libpcap

That's what happens when testing a bunch of stuff that uses libpcap.
Fixes:
http://autobuild.buildroot.net/results/eaa/eaa615830a6b4cf3da88e71315364ad3e380f681/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>