ptpd2: fix static linking

Do the same as in commit b64a06d7
to avoid missing libraries needed by libpcap. Fixes:
http://autobuild.buildroot.net/results/1a1/1a1dadfa2b22074d401b013606f0cede66efe826/



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